A permanent magnetic material

ABSTRACT

There is presented a method for providing a permanent magnetic material comprising hexagonal ferrites, which method does not necessitate neither large magnetic fields nor organic solvents. The produced permanent magnetic materials have excellent properties, in particular in terms of energy product, such as in terms of energy product and density. In further aspects, the invention relates to particles for providing the permanent magnetic material, and a corresponding method of manufacture. In particular embodiments of the invention the hexagonal ferrite is given by CaFe 12 O 19 , SrFe 12 O 19  or BaFe 12 O 19 , such as given SrFe 12 O 19  or BaFe 12 O 19 .

FIELD OF THE INVENTION

The present invention relates to permanent magnetic materials, and inparticular relates to a permanent magnetic material and a correspondingmethod of manufacture.

BACKGROUND OF THE INVENTION

Typically, high performance permanent magnets comprise rare-earthelements, such as neodymium, which may be expensive and/or difficult toaccess. It would be advantageous to provide new permanent magneticmaterials without rare-earth elements, such as magnetic materialswithout rare-earth elements which have improved properties.

Hence, an improved permanent magnetic material that does not includerare-earth elements would be advantageous, and in particular a permanentmagnetic material that does not include rare-earth elements which hasimproved energy product with respect to prior art permanent magneticmaterials that do not include rare-earth elements would be advantageous.

SUMMARY OF THE INVENTION

In particular, it may be seen as an object of the present invention toprovide a magnetic material that solves the above mentioned problems ofthe prior art with being dependent on rare-earth elements. Furthermore,it may be seen as an object of the present invention to provide apermanent magnetic material that does not include rare-earth elementsand which has improved energy product with respect to prior artpermanent magnetic materials that do not include rare-earth elements. Itis a further object of the present invention to provide an alternativeto the prior art.

Thus, the above described object and several other objects are intendedto be obtained in aspects of the invention described below, such aswithin a first aspect of the invention by providing a permanent magneticmaterial and a corresponding method of manufacture.

The invention may be particularly advantageous, such as is particularly,but not exclusively, advantageous for obtaining a method for preparingparticles comprising hexagonal ferrite. According to a first aspect ofthe invention, there is provided a method for preparing particlescomprising hexagonal ferrite for a magnetic material, the methodcomprising

-   -   Forming a precursor solution comprising elements of the        hexagonal ferrite,    -   Feeding the precursor solution, such as a precursor solution        containing a precipitate, into a supercritical reactor, so as to        carry out a supercritical synthesis of the particles, wherein        the particles have an anisotropic shape and wherein the size of        the particles are smaller than or equal to a size enabling        individual particles to become single domain magnets, such as        the size being smaller than 100 nm, such as the size being        smaller than 50 nm.

A possible advantage of this method may be, that it enables providingparticles which may be compacted into a permanent magnetic material,such as compacted into a permanent magnetic material with relativelyhigh energy product, such as without the use of high magnetic fieldand/or organic solvents. Thus, the method enables producing particleswhich facilitate a relatively cheap and safe way of producing permanentmagnetic materials with a high energy product. Another advantage may be,that the method enables dispensing with rare earth elements.

Hexagonal ferrites, also known as hexaferrites, are known in the art,and includes M-type ferrites, such as BaFe₁₂O₁₉ (BaM or barium ferrite),SrFe₁₂O₁₉ (SrM or strontium ferrite), and cobalt-titanium substituted Mferrite, Sr— or BaFe₁₂ _(_) _(2x)Co_(x)Ti_(x)O₁₉ (CoTiM), Z-typeferrites (Ba₃Me₂Fe₂₄O₄₁) such as Ba₃Co₂Fe₂₄O₄₁, or Co_(2Z), Y-typeferrites (Ba₂Me₂Fe₁₂O₂₂), such as Ba₂Co₂Fe₁₂O₂₂, or Co_(2Y), W-typeferrites (BaMe₂Fe₁₆O₂₇), such as BaCo₂Fe₁₆O₂₇, or Co_(2W), X-typeferrites (Ba₂Me₂Fe₂₈O₄₆), such as Ba₂Co₂Fe₂₈O₄₆, or Co_(2X), U-typeferrites (Ba₄Me₂Fe₃₆O₆₀), such as Ba₄Co₂Fe₃₆O₆₀, or Co_(2U). Hexagonalferrites are described in detail in the article “Hexagonal ferrites: Areview of the synthesis, properties and applications of hexaferriteceramics”, Robert C. Pullar, Progress in Materials Science 57 (2012)1191-1334, which reference is hereby incorporated in entirety bereference.

It may furthermore be noted that hexagonal ferrites, also known ashexaferrites, are known in the art, and includes M-type ferrites, suchas BaFe₁₂O₁₉ (BaM or barium ferrite or BaO.6Fe₂O₃), SrFe₁₂O₁₉ (SrM orstrontium ferrite or BaO.6Fe₂O₃), CaFe₁₂O₁₉ (CaM or calcium ferrite orCaO.6Fe₂O₃) and furthermore including particle substitution of the typeBa_(x)Sr_(1-x)Fe₁₂O₁₉ or Ba_(x)Ca_(1-x)Fe₁₂O₁₉ or Ca_(x)Sr_(1-x)Fe₁₂O₁₉or Ca_(y)Ba_(x)Sr_(1-x-y)Fe₁₂O₁₉, and furthermore including, Z-typeferrites (Ba₃Me₂Fe₂₄O₄₁) such as Ba₃Fe₂Fe₂₄O₄₁, or Y-type ferrites(Ba₂Me₂Fe₁₂O₂₂), such as Ba₂Fe₂Fe₁₂O₂₂, or Fe_(2Y), W-type ferrites(BaFe₂Fe₁₆O₂₇), such as BaFe₂Fe₁₆O₂₇, or Fe_(2W), X-type ferrites(Ba₂Me₂Fe₂₈O₄₆), such as Ba₂Fe₂Fe₂₈O₄₆, or Fe_(2X), U-type ferrites(Ba₄Me₂Fe₃₆O₆₀), such as Ba₄Fe₂Fe₃₆O₆₀, or Fe_(2U). Hexagonal ferritesare described in detail in the article “Hexagonal ferrites: A review ofthe synthesis, properties and applications of hexaferrite ceramics”,Robert C. Pullar, Progress in Materials Science 57 (2012) 1191-1334,which reference is hereby incorporated in entirety be reference.

An advantage of employing hexagonal ferrites is that they all have amagnetocrystalline anisotropy (MCA), that is the induced magnetisationhas a preferred orientation within the crystal structure by having aneasy axis of magnetisation (known as uniaxial hexaferrites). Inembodiments of the invention, the hexagonal ferrite has amagnetocrystalline anisotropy, such as an easy axis of magnetization. Inembodiments of the invention, the hexagonal ferrite is an uniaxialferrite.

By ‘anisotropic shape’ may be understood that the particles are notspherical, such as the particles are platelet or plate-like shaped (suchas a length along each of two crystal axes being at least a factor of 2,such as 4, such as 6, such as 8, such as 10 times larger than a lengthalong the third crystal axis), such as shaped as hexagonal-plates. Theparticles may, such as in some embodiments, be at least so anisotropicthat the particles may take on, such as takes on, a preferredorientation during uniaxial pressing of the particles. The particles mayin some embodiments be at least so anisotropic that most particles, suchsubstantially all particles, such as all particles, take a similarorientation, such as substantially the same orientation, such as thesame orientation, during uniaxial pressing of a plurality, such a largenumber, of particles. An advantage of this may be, that alignment may berealized during pressing without necessitating application of anexternal magnetic field.

‘Single domain’ is well known in the art. A single-domain particle maybe defined as one in which the single-domain state has the lowest energyof all possible states.

Throughout this application reference is made to ‘magnetic material’ and‘magnet’. It is understood, that a magnet may be composed of magneticmaterial. In terms of properties, such as energy product, density orcrystallite sizes and orientations, it is further understood, that‘magnet’ and ‘magnetic material’ may be used interchangeably.

In an embodiment of the invention there is provided a method, whereinthe supercritical synthesis comprises heating of the precursor solution,and wherein said heating is achieved by raising the temperature at arate of at least 10° C./second, such as at a rate of at least 20°C./second, such as at a rate of at least 20° C./second. A possibleadvantage of heating of the precursor material at such relatively highrate, e.g., (>10° C./s) may be that it causes a burst of nucleation.

In an embodiment of the invention there is provided a method, wherein areaction time period during the supercritical synthesis is 10 minutes orless, such as 8 minutes or less, such as 6 minutes or less, such as 4minutes or less, such as 2 minutes or less. An possible advantage ofsaid reaction time period being relatively short (e.g., <10 minutes) maybe that it reduces or prevents grain growth.

In an embodiment of the invention there is provided a method, whereinthe hexagonal ferrite comprises XFe₁₂O₁₉, where X is an element selectedfrom the group comprising Calcium (Ca), Strontium (Sr) and Barium (Ba),such as selected from the group consisting of Calcium (Ca), Strontium(Sr) and Barium (Ba), such as from the group comprising Strontium (Sr)and Barium (Ba), such as selected from the group consisting of Strontium(Sr) and Barium (Ba). A possible advantage of employing calcium and/orstrontium, such as CaFe₁₂O₁₉ or SrFe₁₂O₁₉, may be that it comprises onlyelements which are relatively harmless in regards to health andenvironment.

In an embodiment of the invention there is provided a method, wherein astep of forming the precursor solution comprises

-   -   dissolving        -   a compound comprising iron (Fe), such as a compound selected            from the group comprising, such as consisting of:            -   iron nitrate, such as (Fe(NO₃)₃.9H₂O,            -   iron chloride, such as FeCl₃, and            -   iron sulphate, Fe(SO₄)₃,    -   and/or dissolving one or more compounds selected from the group        comprising, such as consisting of:        -   a compound comprising strontium (Sr), such as a compound            selected from the group comprising, such as consisting of:            -   strontium nitrate, such as Sr(NO₃)₂,            -   strontium hydroxide, such as Sr(OH)₂,            -   strontium chloride, such as SrCl₂,        -   a compound comprising Barium (Ba), such as a compound            selected from the group comprising, such as consisting of:            -   barium nitrate, such as Ba(NO₃)₂,            -   barium hydroxide, such as Ba(OH)₂,            -   barium chloride, such as BaCl₂,        -   a compound comprising Calcium (Ca), such as a compound            selected from the group comprising, such as consisting of:            -   calcium nitrate, such as Ca(NO₃)₂,            -   calcium hydroxide, such as Ca(OH)₂,            -   calcium chloride, such as CaCl₂.

In exemplary embodiments, a compound comprising iron (Fe) is dissolvedtogether with any one of compound chosen from the group comprising, suchas consisting of: Sr(OH)₂, Sr(NO₃)₂, and SrCl₂, Ba(OH)₂, Ba(NO₃)₂,BaCl₂.

In an embodiment of the invention there is provided a method, wherein astep of forming the precursor solution comprises

-   -   dissolving        -   a compound comprising iron (Fe), such as a compound selected            from the group comprising, such as consisting of:            -   iron nitrate, such as (Fe(NO₃)₃.9H₂O,            -   iron chloride, such as FeCl₃, and            -   iron sulphate, Fe(SO₄)₃,    -   and/or dissolving        -   a compound comprising strontium (Sr), such as a compound            selected from the group comprising, such as consisting of:            -   strontium nitrate, such as Sr(NO₃)₂,            -   strontium hydroxide, such as Sr(OH)₂,            -   strontium chloride, such as SrCl₂.

In an embodiment of the invention there is provided a method, wherein astep of forming the precursor solution comprises

-   -   dissolving        -   a compound comprising iron (Fe), such as a compound selected            from the group comprising, such as consisting of:            -   iron nitrate, such as (Fe(NO₃)₃.9H₂O,            -   iron chloride, such as FeCl₃, and            -   iron sulphate, Fe(SO₄)₃,    -   and/or dissolving        -   a compound comprising Barium (Ba), such as a compound            selected from the group comprising, such as consisting of:            -   barium nitrate, such as Ba(NO₃)₂,            -   barium hydroxide, such as Ba(OH)₂,            -   barium chloride, such as BaCl₂.

In an embodiment of the invention there is provided a method, wherein astep of forming the precursor solution comprises

-   -   dissolving        -   a compound comprising iron (Fe), such as a compound selected            from the group comprising, such as consisting of:            -   iron nitrate, such as (Fe(NO₃)₃.9H₂O,            -   iron chloride, such as FeCl₃, and            -   iron sulphate, Fe(SO₄)₃,    -   and/or dissolving        -   a compound comprising Calcium (Ca), such as a compound            selected from the group comprising, such as consisting of:            -   calcium nitrate, such as Ca(NO₃)₂,            -   calcium hydroxide, such as Ca(OH)₂,            -   calcium chloride, such as CaCl₂.

In an embodiment of the invention there is provided a method, wherein astep of forming the precursor solution comprises

-   -   dissolving        -   a compound comprising iron (Fe), such as a compound selected            from the group comprising, such as consisting of:            -   iron nitrate, such as (Fe(NO₃)₃.9H₂O,            -   iron chloride, such as FeCl₃, and            -   iron sulphate, Fe(SO₄)₃,    -   and/or dissolving at least two, such as two, compounds selected        from the group comprising, such as consisting of:        -   a compound comprising strontium (Sr), such as a compound            selected from the group comprising, such as consisting of:            -   strontium nitrate, such as Sr(NO₃)₂,            -   strontium hydroxide, such as Sr(OH)₂,            -   strontium chloride, such as SrCl₂,        -   a compound comprising Barium (Ba), such as a compound            selected from the group comprising, such as consisting of:            -   barium nitrate, such as Ba(NO₃)₂,            -   barium hydroxide, such as Ba(OH)₂,            -   barium chloride, such as BaCl₂,        -   a compound comprising Calcium (Ca), such as a compound            selected from the group comprising, such as consisting of:            -   calcium nitrate, such as Ca(NO₃)₂,            -   calcium hydroxide, such as Ca(OH)₂,            -   calcium chloride, such as CaCl₂.

For example, a step of forming the precursor solution comprisesdissolving any one of Fe/Sr/Ba, Fe/Sr/Ca, Fe/Ba/Ca, Fe/Ca/Sr/Ba, Sr/Ba,Sr/Ca, Ba/Ca, Ca/Sr/Ba. It may be understood that a hexagonal ferriteXFe₁₂O₁₉ may thereby be provided, wherein X is a mixture comprising thedissolved species, e.g., Sr/Ba (i.e., Sr and Ba), Sr/Ca (i.e., Sr andCa) or Ba/Ca (Ba and Ca).

In an embodiment of the invention there is provided a method, wherein astep of forming the precursor solution comprises

-   -   dissolving iron nitrate, such as (Fe(NO₃)₃.9H₂O, and strontium        nitrate, such as Sr(NO₃)₂.

In an embodiment of the invention there is provided a method, whereinthe precursor solution has a Sr:Fe ratio of 1:1.

In an embodiment of the invention there is provided a method, whereinthe precursor solution has a X:Fe ratio of 1:1 or R_(x):1 where R_(x) isa number within 0.083-5, such as within ( 1/12)-5, such as within0.083-2, such as within ( 1/12)-2, such as within 0.1-5, such as within0.1-2 (i.e., X being within 1/10 the amount of Fe and 2 times the amountof Fe), such as within 0.125-5, such as within 0.125-2 (i.e., X beingwithin ⅛ the amount of Fe and 2 times the amount of Fe), such as within0.25-1.75, such as within 0.3-1.6, such as within 0.5-1.5, such aswithin 0.7-1.3, such as within 0.8-1.2, such as within 0.9-1.1, such asR_(x) being substantially 1, where X is chosen from the groupcomprising, such as consisting of: Strontium (Sr), Barium (Ba) andCalcium (Ca).

In an embodiment of the invention there is provided a method, whereinthe precursor solution has a Sr:Fe ratio of 1:1 or R_(Sr):1 where R_(Sr)is a number within 0.125-2 (corresponding to the concentration of Srbeing within ⅛ the amount of Fe to 2 times the amount of Fe), such aswithin 0.25-1.75, such as within 0.5-1.5 such as within 0.75-1.25, suchas within 0.9-1.1, such as R_(Sr) being substantially 1, such as R_(Sr)being 1.

In an embodiment of the invention there is provided a method, whereinthe precursor solution has a Ba:Fe ratio of 1:1 or R_(Ba):1 where R_(Ba)is a number within 0.1-2 (corresponding to the concentration of Ba beingwithin 1/10 the amount of Fe to 2 times the amount of Fe), such aswithin 0.25-1.75, such as within 0.5-1.5, such as within 0.75-1.25, suchas within 0.9-1.1, such as R_(Ba) being substantially 1, such as R_(Ba)being 1.

In an embodiment of the invention there is provided a method, whereinthe precursor solution has a Ca:Fe ratio of 1:1 or R_(Ca):1 where R_(Ca)is a number within 0.1-2, such as R_(Ca) being 1.

In an embodiment of the invention there is provided a method, whereinthe method further comprises adding a base, such as an alkalinesolution, to the precursor solution, and wherein a concentration of Fe³⁺iron(III) within the precursor solution when adding the base is within0.05-0.750 M. An advantage of this may be, that it enables controllingthe particle size.

In an embodiment of the invention there is provided a method, whereinthe method further comprises adding a base, such as an alkalinesolution, to the precursor solution, and wherein a concentration of Fe³⁺iron(III) within the precursor solution when adding the base is within0.05-0.750 M, such as within 0.05-0.5, and wherein a final concentrationof the precursor is within 0.05-0.50 M, such as 0.05, and is achievedthrough dilution with base and/or water.

In an embodiment of the invention there is provided a method, wherein analkaline solution is added in a concentration being at least 1.25 times,such as at least 1.50 times, such as at least 2 times, such as 2 times,the concentration of nitrates from both the iron nitrate and thestrontium nitrate. A possible advantage of having the ratio beingrelatively high may be that impurities, such as α-Fe₂O₃ may form atlower values of the ratio.

In an embodiment of the invention there is provided a method, whereinthe alkaline solution comprises a substance selected from the groupcomprising: NaOH, KOH and LiOH, such as from the group comprising NaOHand KOH. In an embodiment of the invention there is provided a method,wherein the alkaline solution comprises a substance selected from thegroup consisting of: NaOH, KOH and LiOH, such as from the groupconsisting of: NaOH and KOH. A possible advantage of employing thesesubstances may be that they ensure that particles of correct compositionare formed. An advantage of employing NaOH or KOH may be that they yielda high purity.

In an embodiment of the invention there is provided a method, whereinthe alkaline solution is added drop wise under constant stirring until adark red precipitate is formed.

In an embodiment of the invention there is provided a method comprisingfeeding the precursor solution, such as the precursor solutioncontaining the dark red precipitate, into a supercritical reactor.

In an embodiment of the invention there is provided a method, whereinthe precursor solution is fed into the supercritical reactor at the flowrate of within 0.5-50 mL/min, such as within 1-10 mL/min, such as 5mL/min.

In an embodiment of the invention there is provided a method comprisingfeeding deionized water into the supercritical reactor at a flow rate ofwithin 0.15-150 mL/min, such as within 3-30 mL/min, such as 15 mL/min.In and embodiment, the method comprises feeding the deionized water intothe supercritical reactor from a second line.

In an embodiment of the invention there is provided a method comprising

-   -   feeding the precursor solution, such as the precursor solution        containing the dark red precipitate, into the supercritical        reactor, at a first flow rate,    -   feeding deionized water into the supercritical reactor at a        second flow rate,    -   wherein the ratio of the first flow rate and the second flow        rate is between 1:0.3 and 1:30, such as between 1:1 and 1:10,        such as 1:3.

In an embodiment of the invention there is provided a method, whereinthe precursor solution and the deionized water meet at a mixing point.

According to a second aspect of the invention, there is providedparticles comprising hexagonal ferrite for a magnetic material, such asparticles prepared according to the first aspect, wherein the particleshave an anisotropic shape and wherein the size of the particles aresmaller than or equal to a size enabling individual particles to becomesingle domain magnets, such as the particles being smaller than 100 nm,such as the particles being smaller than 50 nm.

In an embodiment of the invention there is provided particles, whereinthe hexagonal ferrite comprises XFe₁₂O₁₉, where X is an element selectedfrom the group comprising Calcium (Ca), Strontium (Sr) and Barium (Ba),such as selected from the group comprising Strontium (Sr) and Barium(Ba). In an embodiment of the invention there is provided particles,wherein the hexagonal ferrite comprises XFe₁₂O₁₉, where X is an elementselected from the group consisting of: Calcium (Ca), Strontium (Sr) andBarium (Ba), such as selected from the group consisting of: Strontium(Sr) and Barium (Ba).

When referring to “size”, as such, it may be understood that the size‘as such’ refers to the greatest distance between any two points of aparticle.

In an embodiment of the invention there is provided particles, whereinthe hexagonal ferrite comprises XFe₁₂O₁₉, where X is combinationcomprising, such as consisting of, two or three elements selected fromthe group comprising, such as consisting of, Calcium (Ca), Strontium(Sr) and Barium (Ba), such as any one of Ca/Sr, Ca/Ba, Sr/Ba, orCa/Sr/Ba.

In an embodiment of the invention there is provided particles, whereinthe dimensions of the particles may be described by dimensions along afirst crystal axis (a-axis), a second crystal axis (b-axis) and a thirdcrystal axis (c-axis), and wherein the dimensions of the particles aresubstantially larger along the first crystal axis (a-axis) and/or thesecond crystal axis (b-axis) relative to the dimension along the thirdcrystal axis (c-axis).

In an embodiment of the invention there is provided particles, whereinthe dimensions of the particles are at least 2 times larger, such as atleast 3 times larger, such as at least 4 times larger, such as at least5 times larger, such as at least 10 times larger, such as substantially10 times larger, along the first crystal axis (a-axis) and/or the secondcrystal axis (b-axis) relative to the dimension along the third crystalaxis (c-axis). The ratio between the largest lateral dimension (alongthe first and second crystal axis) and the dimension orthogonal thereto(along the third crystal axis) may be referred to as aspect ratio. Apossible advantage of having a larger aspect ratio may be that itenables the properties of a permanent magnetic material made frompressing or compacting the particles, to be better. In an embodiment,the particles are plate like, such as both of the dimensions along thefirst and second crystal axis are substantially larger, such as at least2 times larger, such as at least 5 times larger, such as at least 10times larger, such as substantially 10 times larger, than the dimensionalong the third crystal axis.

In an embodiment of the invention there is provided particles, wherein adimension of the particles along the first crystal axis (a-axis) iswithin 20-40 nm, such as substantially 30 nm, such as 30 nm, and whereina dimension along the second crystal axis (b-axis) is within 20-40 nm,such as substantially 30 nm, such as 30 nm, and wherein a dimensionalong a third crystal axis (c-axis) is within 2-4 nm, such assubstantially 3 nm, such as 3 nm. In an alternative formulation, thesize of the particles may be described as being given by[20-40]×[20-40]×[2-4] nm³, such as 30×30×3 nm³. Particles of this sizemay during a compaction, such as a pressing, grow into sizes, such as[a×b×c]=60×60×12 nm³.

In an embodiment of the invention there is provided particles, wherein adimension of the particles along the first crystal axis (a-axis) issubstantially 30 nm and wherein a dimension along the second crystalaxis (b-axis) is substantially 30 nm and wherein a dimension along athird crystal axis (c-axis) is substantially 3 nm. In an alternativeformulation, the size of the particles may be described as being givenby 30×30×3 nm³.

In an embodiment of the invention there is provided particles, whereinthe energy product (BH_(max)) of the particles, such as the as preparedparticles, is at least 0.1 kJ/m³, such as at least 1.0 kJ/m³, such assubstantially 1 kJ/m³. By ‘as prepared particles’ is understoodparticles which have not been compacted, such as particles which havenot been subjected to pressure substantially above atmospheric pressure,such as particles which have not been subjected to pressuresubstantially above atmospheric pressure and/or a temperaturesubstantially above room temperature.

According to a third aspect of the invention, there is provided a methodfor preparing a permanent magnetic material comprising hexagonalferrite, the method comprising, such as comprising the steps of, such ascomprising the successive steps of:

-   -   obtaining particles comprising hexagonal ferrite, such as        particles prepared according to the first aspect and/or provided        according to the second aspect, which particles have an        anisotropic shape, such as an anisotropic shape and a size being        smaller than or equal to a size enabling individual particles to        become single domain magnets, such as the size enabling        individual particles to become single domain magnets, such as        smaller than 100 nm, such as the particles being smaller than 50        nm,    -   compacting the particles into a permanent magnetic material,        such as the step of compacting the particles into a permanent        magnetic material comprises applying a uniaxial pressing step,        wherein the step of compacting the particles comprises applying        a pressure above atmospheric pressure and a temperature above        room temperature, such as said temperature being above a        blocking temperature of said particles, such as said pressure        and temperature being applied in a temporally overlapping manner        (such as at least a portion of the pressing above atmospheric        pressure is temporally overlapping with at least a portion of        the temperature being above room temperature, such as at least a        blocking temperature of said particles), such as substantially        simultaneously, such as simultaneously, and wherein a size of        the particles after the step of compacting are smaller than or        equal to a size enabling individual particles to become single        domain magnets, such as the size enabling individual particles        to become single domain magnets.

An advantage of the present method may be, that it enables preparing apermanent magnetic material, such as a permanent magnetic materialcomprising multiple particles comprising hexagonal ferrite, such as ahigh quality permanent magnetic material, which method may be seen assimple, cost efficient, cheap and/or environmentally friendly. Forexample, the method may facilitate dispensing with a need for applyingan external magnetic field during compacting. For example, the methodmay facilitate dispensing with a need for employing glue and/or solventsduring compacting. By ‘compacting’ may be understood a process oftransforming a plurality of particles into a single, coherent materialcomprising said plurality of particles. It may in general be understood,that the process of compacting may last less than 20 minutes, such asless than 15, such as less than 10 minutes, such as 9 minutes or less.An advantage of a relatively short period of compaction, may be thatgrain growth is accordingly limited. It may in general be understood,that the temperature above room temperature may be at least a blockingtemperature of said particles. It may in general be understood, that thepressure above atmospheric pressure suffice in order to make theparticles adhere to each other and obtain a preferential orientation atthe temperature above room temperature.

In an embodiment of the invention there is provided a method, whereinthe hexagonal ferrite comprises XFe₁₂O₁₉, where X is an element selectedfrom the group comprising Calcium (Ca), Strontium (Sr) and Barium (Ba),such as from the group consisting of Calcium (Ca), Strontium (Sr) andBarium (Ba).

In an embodiment of the invention there is provided a method, wherein

-   -   obtaining particles comprising hexagonal ferrite, which        particles have an anisotropic shape,        comprises obtaining particles comprising hexagonal ferrite,        where the anisotropic shape is a plate like shape, and wherein        the size of the particles (such as the size of the particles        before initiation of compacting the particles) is at most 100        nm, such as at most 75 nm, such as at most 50 nm, such as at        most 40 nm, such as the sizes along the crystal axes [a; b; c]        being within [20-40 nm; 20-40 nm; 2-4 nm].

In an embodiment of the invention there is provided a method, whereinthe method comprises reducing or breaking a magnetic interaction betweenthe particles when compacting the particles and/or during compacting theparticles, so as to allow alignment of the particles when compacting theparticles and/or during compacting the particles. This may be seen asadvantageous, since it facilitates alignment of the particles, since amagnetic interaction between the particles may impede the alignment. Themagnetic interaction may be reduced or broken by supplying sonic and/orthermal energy, such as by supplying sufficient thermal energy forexceeding the blocking temperature of the particles. An advantage ofthis may be that it enables a method for preparing a permanent magneticmaterial without using glue and/or solvents. In an embodiment of theinvention there is provided a method, wherein the method does notcomprise having glue and/or solvent between the particles duringcompacting the particles.

In an embodiment of the invention there is provided a method, whereinthe method comprises:

-   -   pre-heating of the particles, wherein said pre-heating comprises        applying a pre-heating temperature above room temperature to        said particles before compacting the particles, so that a        temperature of said particles is the pre-heating temperature        when the compacting is initiated.

In an embodiment, the step of compacting the particles is carried outwithout an external magnetic field applied, such within a magnetic fieldbeing 0 T. In an embodiment, the step of compacting the particles iscarried out in an external magnetic field applied which is within therange 0-1 T, such as within 0-0.5 T, such as within 0-0.25 T, such aswithin 0-0.1 T, such as within 0-0.05 T.

In an embodiment of the invention there is provided a method forpreparing a permanent magnetic material comprising hexagonal ferrite,wherein the hexagonal ferrite comprises XFe₁₂O₁₉, where X is an elementselected from the group comprising Calcium (Ca), Strontium (Sr) andBarium (Ba), such as selected from the group comprising Strontium (Sr)and Barium (Ba). In an embodiment of the invention there is provided amethod for preparing a permanent magnetic material comprising hexagonalferrite, wherein the hexagonal ferrite comprises XFe₁₂O₁₉, where X is anelement selected from the group consisting of: Calcium (Ca), Strontium(Sr) and Barium (Ba), such as selected from the group consisting of:Strontium (Sr) and Barium (Ba).

In an embodiment of the invention there is provided a method forpreparing a permanent magnetic material comprising hexagonal ferritewherein the obtained particles are smaller compared to the particles inthe permanent magnetic material, such as wherein the particles areenlarged during the step of compacting the particles.

In an embodiment of the invention there is provided a method forpreparing a permanent magnetic material, wherein the step of compactingthe particles comprises uniaxial pressing. It may be understood that inuniaxial pressing, the pressure is applied along an axis, such as a1-dimensional axis. The pressing may be carried out by placing theparticles between two punches and moving one or both punches along saidaxis, such as towards each other in opposite directions, so as to applya pressure on the particles, such as placing the particles, such as inthe form of a powder, between two punches and applying a pressure viathe punches along a single direction. A possible advantage of uniaxialpressing may be, that it enables alignment of the anisotropic particles.

In a particular embodiment, the step of compacting the particlescomprises uniaxial hot pressing. Uniaxial hot pressing may alternativelybe phrased hot uniaxial pressing (HUP). It is understood that inuniaxial hot pressing, heat is supplied to the particles duringpressing. In a particular embodiment, the step of supplying heatcomprises a step chosen from the group comprising, such as consistingof:

-   -   resistive heating, such as applying a current through the        particles, such as applying a pulsed DC current through the        particles,    -   induction heating,    -   heating via a flux of incoming electromagnetic radiation, such        as heating via light, such as via LASER,    -   microwave heating.

In an embodiment of the invention there is provided a method forpreparing a permanent magnetic material, wherein the step of compactingthe particles comprises Spark Plasma Sintering (SPS), such as the methodcomprising loading the particles into a pressing tool (such as agraphite pressing tool) wherein uniaxial pressure is applied to punchesand a pulsed DC current is directed through the pressing tool and theparticles. A possible advantage of employing SPS, may be that it enablescompacting particles into a permanent magnetic material without havingto use a large external magnetic field and/or organic solvents. Anotherpossible advantage of SPS is that it enables fast heating of theparticles, which in turn enable that particles are heated above ablocking temperature so fast that the particles do not have enough timeto grow too large.

In an embodiment of the invention there is provided a method forpreparing a permanent magnetic material wherein the pressure is at least20 MPa, such as at least 40 MPa, such as at least 60 MPa, such as atleast 80 MPa, such as 80 MPa, such a below 100 MPa, such as between 20MPa and 100 MPa. A possible advantage of employing relatively largepressure may be that it may enable yielding a high density of the finalpermanent magnetic material.

In an embodiment of the invention there is provided a method forpreparing a permanent magnetic material, wherein the change from roomtemperature to said temperature above room temperature, such as ablocking temperature, may comprise temperature changes at a rate of atleast 10° C./minute, such as at least 25° C./minute, such as at least50° C./minute, such as at least 75° C./minute, such as at least 100°C./minute, such as substantially 100° C./minute.

In an embodiment of the invention there is provided a method forpreparing a permanent magnetic material, wherein the direct pulsed DCcurrent is applied so as to heat the particles at a rate of at least 10°C./minute, such as at least 25° C./minute, such as at least 50°C./minute, such as at least 75° C./minute, such as at least 100°C./minute, such as substantially 100° C./minute.

In an embodiment of the invention there is provided a method forpreparing a permanent magnetic material, wherein the direct pulsed DCcurrent is applied so as to heat the particles to a temperature of atleast 800° C., such as at least 875° C., such as 950° C., such as below1450° C., such as between 800° C. and 1450° C. A possible advantage ofemploying relatively high temperature may be that it enables sinteringthe particles. Another possible advantage of heating to a relativelyhigh temperature, such as heating to or above a magnetic phasetransition, may be that it reduces or breaks magnetic adhesion of theparticles. Breakage of the magnetic adhesion may in turn aid alignmentof the particles under the applied pressure.

In an embodiment of the invention there is provided a method forpreparing a permanent magnetic material, wherein at least partiallyduring compacting the particles into a permanent magnetic material

-   -   the particles are heated to a temperature of at least 800° C.,        such as at least 875° C., such as 950° C., such as between        800° C. and 1450° C., and    -   the pressure is at least 20 MPa, such as at least 40 MPa, such        as at least 60 MPa, such as at least 80 MPa, such as 80 MPa,        such a below 100 MPa, such as between 20 MPa and 100 MPa.

An advantage of having both relatively high temperature and relativelyhigh pressure during compacting of the particles, may be that it enablesimproving the alignment of the particles, since the relatively hightemperature may reduce or break a magnetic interaction between theparticles, and the applied pressure may then ensure that the anisotropicparticles align, thus the temperature and pressure may worksynergistically together to improve alignment. It may in general beunderstood, that the temperature above room temperature may be at leasta blocking temperature of said particles.

In an embodiment of the invention there is provided a method forpreparing a permanent magnetic material, wherein the temperature aboveroom temperature is held for at least 1 minute, such as at least 2minutes, such as substantially 2 minutes, such as at least 5 minutes,such as at least 10 minutes, before cooling to room temperature. Apossible advantage of employing relatively long time may be that itenables improved properties of the final permanent magnetic material,such as through increasing a size of the particles, such as the grainsize of the particles.

In an embodiment of the invention there is provided a method forpreparing a permanent magnetic material, wherein the step of obtainingparticles, comprises

-   -   preparing particles according to the first aspect, or    -   obtaining particles according to the second aspect.

In an embodiment of the invention there is provided a method forpreparing a permanent magnetic material, wherein the method furthercomprises annealing the permanent magnetic material, such as annealingfor at least 1 hour, such as at least 2 hours, such as at least 4 hours,such as 4 hours, such as between 1-10 hours, such as between 2-6 hours,such as between 2.5-5 hours, such as 4 hours, such as annealing at atemperature of between 800-1200° C., such as annealing at a temperatureof between 800-1000° C., such as at a temperature of 850° C., such asannealing for 4 hours at 850° C. A possible advantage of annealing maybe that it improves the properties, such as the energy product of thepermanent magnetic material. In a specific embodiment, the annealing maybe preceded by a heating step, such as heating within 1 hour from 750°C. to 850° C. Another advantage of annealing may be that it renders itpossible to obtain particles which are relatively small, such as smallerthan size enabling individual particles to become single domain magnets(which entails relatively low blocking temperature), exceeding therelatively low blocking temperature (which may be realized fast and/orin simple equipment since the blocking temperature is relatively low),compacting the particles (which will then be relatively small, such astoo small to be single domain magnets), annealing so as to increase theparticle size into a size enabling individual particles to become singledomain magnets. Thus, besides enabling optimization, annealing alsoenables utilizing of otherwise too small particles and optionally gainbenefits from using such small particles.

According to a fourth aspect of the invention, there is provided apermanent magnetic material comprising particles comprising hexagonalferrite, such as a permanent magnetic material prepared according to thethird aspect, wherein a size of the particles are smaller than or equalto a size enabling individual particles to become single domain magnets,such as the size enabling individual particles to become single domainmagnets.

When referring to a permanent magnetic material, such as within thefourth aspect, it may be understood, that the permanent magneticmaterial is a multi-grain material, such as a macroscopically sizedmaterial, such as the material being at least 1 mm³, such as at least 1cm³.

In an embodiment of the invention there is provided a permanent magneticmaterial, wherein the particles have an anisotropic shape, such as theparticles having a plate like shape, such as hexagonal plate like shape.

In an embodiment of the invention there is provided a permanent magneticmaterial, wherein a size of the particles are smaller than or equal to asize enabling individual particles to become single domain magnets, suchas a size enabling individual particles to become single domain magnets,such as 100 nm or less, such as 50 nm or less,

and wherein the particles have an anisotropic shape, such as theparticles having a plate like shape, such as the particles having anaspect ratio of 2 or more, such as having an aspect ratio of 5 or more,such as having an aspect ratio of 10 or more,and wherein crystallites in the permanent magnetic material have apreferential orientation, such as a texture index of at least 2.

In an embodiment of the invention there is provided a permanent magneticmaterial, wherein the hexagonal ferrite in the permanent magneticmaterial occupies at least 90 vol % of the volume, such as at least 93vol %, such as at least 95 vol %, such as at least 96 vol %, such as atleast 97 vol %. An advantage of having the hexagonal ferrite occupying alarge volume percentage may be, that it enables better magneticproperties, such as a higher energy product and/or energy density, sinceless space is wasted around the hexagonal ferrite material.

In an embodiment of the invention there is provided a permanent magneticmaterial, wherein impurities, such as impurities being material notbeing said hexagonal ferrite, in the permanent magnetic materialcontribute to less than 3 wt %, such as less than 2.5 wt %, such as lessthan 2 wt %, such as less than 1.5 wt %, such as less than 1 wt %, suchas less than 0.5 wt %, such as less than 0.25 wt %, such as less than0.1 wt %. ‘Impurities’ may be understood to relate to any material otherthan the hexagonal ferrite. For example, glue and/or solvent residuesmay be considered impurities. An advantage of having no or only arelatively small amount of impurities may be that it enables bettermagnetic properties, such as a higher energy product and/or energydensity, since less space is wasted on impurities.

In an embodiment of the invention there is provided a permanent magneticmaterial, wherein dimensions of the particles are at least 2 timeslarger, along a first crystal axis (a-axis) and/or a second crystal axis(b-axis) relative to a dimension along a third crystal axis (c-axis).

In general, a plate like shape may be understood to be a shape whereinboth of the dimensions along the first and second crystal axis aresubstantially larger, such as at least 2 times larger, such as at least5 times larger, such as at least 10 times larger, such as substantially10 times larger, than the dimension along the third crystal axis.

In an embodiment of the invention there is provided a permanent magneticmaterial, wherein the hexagonal ferrite comprises XFe₁₂O₁₉, where X isan element selected from the group comprising Calcium (Ca), Strontium(Sr) and Barium (Ba), such as selected from the group comprisingStrontium (Sr) and Barium (Ba). In an embodiment of the invention thereis provided a permanent magnetic material, wherein the hexagonal ferritecomprises XFe₁₂O₁₉, where X is an element selected from the groupconsisting of: Calcium (Ca), Strontium (Sr) and Barium (Ba), such asselected from the group consisting of: Strontium (Sr) and Barium (Ba).

In an embodiment of the invention there is provided a permanent magneticmaterial, wherein crystallites in the permanent magnetic material have apreferential orientation, such as the crystallites in the permanentmagnetic material being substantially aligned, such as the crystallitesin the permanent magnetic material being aligned. An advantage of thismay be that it enables providing a collection of oriented single domainnanoparticles, which may be beneficial for producing a larger netmagnetisation than non-oriented particles or a bulk material containingmultiple magnetic domains.

In an embodiment of the invention there is provided a permanent magneticmaterial, wherein a texture index of the permanent magnetic material isat least 2, such as at least 2.5, such as at least 3.0, such as at least3.25, such as at least 3.5, such as at least 4.0, such as at least 4.5,such as at least 5, such as at least 6, such as at least 7, such as atleast 8, such as at least 9, such as at least 10, such as at least 11,such as at least 12, such as at least 15, such as at least 17, such asat least 17.2, such as 17.2.

Texture index is well known in the art and is to be understood as isknown in the art. A description of the texture index can for example befound in “Texture Analysis in Materials Science—Mathematical Methods”,Bunge H-J, (1982), (London: Butterworths), which is hereby incorporatedby reference in entirety, and section “4.8 Texture index” is inparticular incorporated by reference.

In an embodiment of the invention there is provided a permanent magneticmaterial, wherein a ratio J_(r)(0°)/J_(r)(90°) is at least is at least2, such as at least 2.5, such as at least 3.0, such as at least 3.5,such as at least 4.0, such as at least 4.35, such as 4.35, wherein saidratio J_(r)(0°)/J_(r)(90°) is a ratio between

-   -   a first remanence value J_(r)(0°) obtained at a first        orientation of the permanent magnetic material with respect to        an external magnetic applied field,    -   a second remanence value J_(r)(90°) obtained at a second        orientation of the permanent magnetic material with respect to        the applied external magnetic field, wherein the second        orientation is orthogonal to the first orientation.

Any one of the first remanence value and the second remanence value maybe obtained, such as measured, using a vibrating sample magnetometer(VSM), such as a vibrating sample magnetometer as is known in the art.It may be understood that the first orientation may be an orientationwherein an easy axis of the permanent magnetic material is aligned, suchas is parallel, with the applied external magnetic field and/or that thesecond orientation may be an orientation wherein a hard magnetic axis ofthe permanent magnetic material is aligned, such as is parallel, withthe applied external magnetic field.

In an embodiment of the invention there is provided a permanent magneticmaterial, wherein the relative orientation of crystallites within thepermanent magnetic material is at least 10%, such as at least 20%, suchas at least 30%, such as at least 40%, such as at least 45%, such as55%, such as the relative orientation being determined by the method of:

-   -   a. obtaining a powder X-ray diffraction pattern of the permanent        magnetic material,    -   b. sum the intensities for the reflections along the        00l-direction starting with 004 and including 006, 008, 0010,        0012 and 0014 for the first 82 directions excluding 002        diffraction planes, i.e., obtain the value ΣI(00l) for the first        82 directions excluding 002 diffraction planes,    -   c. sum the intensities for all which are within the first 82        directions excluding 002, i.e., obtain the value ΣI(hkl) for the        first 82 directions excluding 002, starting at the diffraction        plane 004 and adding all reflection planes until hkl=315 giving        a total of 81 reflections    -   d. calculate the relative orientation as the ratio        ΣI(00l)/ΣI(hkl).

This method for quantifying the relative orientation will be referred toas the relative orientation quantification method.

It is understood that the relative orientation may be given as a ratioor a percentage, for example the ratio of 1:2=0.5 may be given inpercentage as 50%.

In an embodiment of the invention there is provided a permanent magneticmaterial, wherein the preferential orientation is with c-axis latticeplanes substantially parallel, such as parallel to each other, such assubstantially orthogonal, such as orthogonal to a pressing directionemployed during a preparation of the permanent magnet. It may beunderstood that in some embodiments, crystallites with c-axis latticeplanes which are aligned less than 25% from a direction being orthogonalto a pressing direction are construed to be substantially orthogonal tothe pressing direction. It is understood, that these crystallites mayalso to some extent contribute to a magnetization along the c-axis.

In an embodiment of the invention there is provided a permanent magneticmaterial, wherein crystallites in the permanent magnetic material have alength along a third crystal axis (c-axis) of less than 250 nm, such asless than 200 nm, such as less than 150 nm, such as less than 100 nm,such as less than 75 nm, such as less than 50 nm, such as within 2 nmand 150 nm, such as within 2 nm and 100 nm, such as within 2 nm and 75nm, such as within 3 nm and 150 nm, such as within 3 nm and 100 nm, suchas within 3 nm and 75 nm, such as within 25 nm and 75 nm, such as within50 nm and 70 nm, such as approximately 60 nm. A possible advantage ofsuch dimension may be that it enables that individual particlescorrespond to single domain magnets. Another possible advantage of suchdimension may be that it enables that individual particles correspond tosingle domain magnets and have a relatively low blocking temperature,which may allow the magnetic adhesion to be reduced or broken atrelatively low temperatures, such as during a compaction of a powder ofparticles into the permanent magnetic material, such as when performingan SPS pressing. The nanoparticles have been made with sizes rangingfrom 20 nm to 70 nm along the a/b axis and from 2.4 nm to 11 nm alongthe c axis by varying the concentration of starting precursor materialin the solution.

In an embodiment of the invention there is provided a permanent magneticmaterial, wherein crystallites in the permanent magnetic material have alength along a first crystal axis (a-axis) and/or second crystal axis(b-axis) of less than 250 nm, such as within 25-250 nm, such as lessthan 200 nm, such as less than 175 nm, such as less than 150 nm, such asless than 100 nm, such as less than 80 nm, such as less than 50 nm, suchas within 30 nm and 175 nm, such as within 50 nm and 90 nm, such aswithin 60 nm and 80 nm, such as within 65 nm and 75 nm, such asapproximately 70 nm. A possible advantage of such dimension may be thatit enables that individual particles correspond to single domainmagnets. Another possible advantage of such dimension may be that itenables that individual particles correspond to single domain magnetsand have a relatively low blocking temperature, which may allow themagnetic adhesion to be reduced or broken at relatively lowtemperatures, such as during a compaction of a powder of particles intothe permanent magnetic material, such as when performing an SPSpressing. In general, the crystallites in the permanent magneticmaterial may have a length along a first crystal axis (a-axis) and/orsecond crystal axis (b-axis) of 25 nm or more, which lower limit may becombined with any upper limit referred to above, such as within 25-250nm. An advantage of this lower limit may be that it enables individualparticles to form single-domain particles.

In an embodiment of the invention there is provided a permanent magneticmaterial, wherein the permanent magnetic material has an energy product(BH_(max)) of more than 11 kJ/m³, such as more than 15 kJ/m³, such asmore than kJ/m³, such as more than 25 kJ/m³, such as at least 26 kJ/m³,such as 26 kJ/m³, such as at least 28.5 kJ/m³, such as 28.5 kJ/m³.

In an embodiment of the invention there is provided a permanent magneticmaterial, wherein the permanent magnetic material is shaped into apermanent magnet which has a diameter of 8 mm.

In an embodiment of the invention there is provided a permanent magneticmaterial, wherein the permanent magnetic material is shaped into apermanent magnet which has a thickness of 1 mm.

In an embodiment of the invention there is provided a permanent magneticmaterial, wherein the permanent magnetic material is shaped into apermanent magnet which has a diameter of at least 5 mm. In an embodimentof the invention there is provided a permanent magnetic material,wherein the permanent magnetic material is shaped into a permanentmagnet which has a thickness of at least 1 mm. In an embodiment of theinvention there is provided a permanent magnetic material, wherein thepermanent magnetic material is shaped into a permanent magnet which hasa diameter of at least 1 mm and wherein the permanent magnetic materialis shaped into a permanent magnet which has a thickness of at least 0.1mm. In an embodiment of the invention there is provided a permanentmagnetic material, wherein the permanent magnetic material is shapedinto a permanent magnet with a volume of at least 1 mm³, such as atleast 2, 5, 10, 20, 50, 100, 200, 500, 1000, 10000 or 100000 mm³.

In an embodiment of the invention there is provided a permanent magneticmaterial, wherein the permanent magnetic material has a density of atleast 2.0 g/cm³, such as at least at least 3.0 g/cm³, such as at leastat least 4.0 g/cm³, such as at least 4.5 g/cm³, such as at least 4.7g/cm³, such as 4.7 g/cm³, such as at least at least 5.0 g/cm³, such asat least at least 5.2 g/cm³, such as substantially 5.3 g/cm³. Anadvantage of having a high density, such as at least 5.2 g/cm³, may bethat the energy product may be higher. An advantage of having a highdensity may be that it enables a higher energy density.

In an embodiment of the invention there is provided a permanent magneticmaterial, which has little or no glue and/or little or no solventbetween the particles. This may be realized by manufacturing accordingto the third aspect. An advantage of this may be that it enables arelatively high density of the magnetic material, such as a materialwhere little space is wasted on, e.g., glue and/or solvent.

In an embodiment of the invention there is provided a device forinter-converting between electrical energy and kinetic energy, whereinthe device comprises the permanent magnetic material, such as thepermanent magnetic material prepared according to the fourth aspect. Inanother embodiment of the invention there is provided a device forinter-converting between electrical energy and kinetic energy, whereinthe device comprises permanent magnetic material provided according tothe third aspect.

The applications for such a device range is from kitchen appliances togearless wind turbines and electrical cars. Permanent magnets are thekey component in electro motors and dynamos, where the permanentmagnets, e.g., together with a current conducting Cu coil, isresponsible for the inter-conversion between electricity and motion.

The first, second and third and fourth aspect of the present inventionmay each be combined with any of the other aspects. These and otheraspects of the invention will be apparent from and elucidated withreference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE FIGURES

The permanent magnetic material and corresponding method of manufactureand the particles and corresponding method of manufacture according tothe invention will now be described in more detail with regard to theaccompanying figures. The figures show one way of implementing thepresent invention and is not to be construed as being limiting to otherpossible embodiments falling within the scope of the attached claim set.

FIG. 1 illustrates the design of a permanent magnet on a logarithmicscale.

FIG. 2 shows illustrations of the structure of a single domain magneticnanocrystal.

FIGS. 3(a)-(b) show photographs of a nanopowder.

FIG. 4 shows a schematic diagram of the hydrothermal flow synthesissetup.

FIGS. 5-6 show TEM and AFM images of nanoparticles of SrFe₁₂O₁₉.

FIG. 7 shows the XRPD pattern of the supercritical synthesized sample.

FIGS. 8-9 shows a hysteresis loop of as prepared SrFe₁₂O₁₉,respectively, hysteresis loops for different samples.

FIG. 10 is a schematic illustration of the working principles of a SPSpress.

FIG. 11 illustrates platelets exposed to elevated pressure andtemperature.

FIG. 12 shows normalized powder diffraction patterns.

FIGS. 13-14 show X-ray date corresponding to the magnetic material.

FIG. 15 shows the effect on the energy product of annealing.

FIG. 16 shows an image of an SrFe₁₂O₁₉ powder.

FIG. 17 shows a graph of particle size as a function of concentration ofFe(NO₃)₃.9H₂O and Sr(NO₃)₂ when addition of the base is taking place.

FIG. 18 shows a graph of particle size as a function of the ratio ofBa:Fe.

FIG. 19 is a schematic of the pole figure measurement.

FIG. 20 shows the volume fraction of samples with a given alignment.

FIG. 21 shows reduced pole figures.

FIG. 22 shows the sample setup in the VSM.

FIG. 23 shows hysteresis curves for various samples.

DETAILED DESCRIPTION OF AN EMBODIMENT Structural Control fromSub-Nanometer to Bulk

The design of permanent magnets is challenging as it involves structuralcontrol at all levels from atomic positions to the macroscopicarrangement of nanoparticles with a specific size and shape. On thesub-nanometer scale ferromagnetism predominantly originates from theself-rotation of unpaired electrons in atoms and quantum mechanicalinteractions may cause the magnetic spin of many atoms to align withrespect to each other. For small particles all spins can be easilyrotated by thermal energy and such compounds are known assuperparamagnetic. At a specific size, which depends on the material,the spins become increasingly difficult to rotate and the particlebecomes a single domain magnet Nanoparticles smaller than 25 nm areoften superparamagnetic, while particles in the range of 25-250 nm aresingle domain magnets. Upon further size increase multiple domains withdifferent magnetic orientations are introduced and this reduces themagnetic energy of the system. Thus, a collection of oriented singledomain nanoparticles can produce a larger net magnetisation than a bulkmaterial containing multiple magnetic domains. The challenge of theresearch project can be divided into different size domains: atomic,nanometre and micrometre length scales. At the atomic scale the goal isto synthesize structures giving rise to strong quantum mechanicalinteractions aligning the magnetic spins. The next step is to ensurethat these atomic structures are reproduced in nanoparticles ofappropriate size and shape. Finally, a large number of nanoparticleswith perfected atomic structure, size, and shape must be compacted intoa bulk material, where the individual nanoparticles are ordered withrespect to each other. Control at all length scales is beneficial formaking such high performance permanent magnets.

Thus, according to an embodiment of the invention there is provided amethod for providing a permanent magnetic material, such as a magnetcomprising XFe₁₂O₁₉ where X is an element selected from the groupcomprising, such as consisting of: Calcium (Ca), Strontium (Sr) andBarium (Ba), such as selected from the group comprising, such asconsisting of: Strontium (Sr) and Barium (Ba), the method comprising thesteps of

-   -   synthesizing structures giving rise to interactions, such as        strong quantum mechanical interactions, aligning the magnetic        spins, such as via providing structures exhibiting        magnetocrystalline anisotropy,    -   ensuring that these atomic structures are reproduced in        nanoparticles of appropriate size, such as enabling the        nanoparticles to correspond to a size being smaller than or        equal to a size enabling the individual particles to become a        single magnetic domain, and appropriate shape, such as the shape        being anisotropic, such as platelet shaped, such as the size and        shape facilitating a breakage of magnetic interactions between        said particles upon a heating of said particles so as to        facilitate alignment of the particles,    -   compacting the particles into a bulk material, where the        individual nanoparticles are ordered with respect to each other,        optionally aided by uniaxial pressure and/or elevated        temperature.

FIG. 1 illustrates the design of a permanent magnet on a logarithmicscale from femtometres to centimetres. Starting from the left is shownan unpaired electron 102 spinning, which gives rise to the magneticmoment. Next an iron atom 104 is coordinated to oxygen and thisstructure is placed in a nanometre sized unit cell 106. The size andshape of the nanoparticles are controlled to create a single domainmagnetic particle 108 measuring 25-250 nm. These single domain particlesare compacted into micrometre sized particles 110 with their magneticaxis pointing along the same direction. The final magnetic sample 112 isdepicted in the illustration as being in the mm-cm range, but may inother embodiments be smaller or larger.

FIG. 2 shows illustrations of the structure of a single domain magneticnanocrystal.

FIG. 2(a) shows a structure of a single domain magnetic nanocrystal 208.The structure may be probed with an X-ray powder diffraction instrument,such as a Rigaku X-ray diffractometer, wherein the X-ray source ismounted on one side of the sample (which is found in the middle), whilethe detector is on the other side. The emitted X-rays are reflected bythe atomic planes in the nanocrystals and the angular positions andintensities of these reflections gives information about theirstructures at the atomic level.

FIG. 2(b) shows the extracted size and shape of the nanoparticles fromthe Rietveld refinements, i.e., the approximate dimensions extractedfrom the powder diffraction model, cf., FIG. 7, which represents anaverage in contrast to the nanoparticles shown in FIGS. 5-6, whichdepict individual nanoparticles.

FIG. 3(a) shows a photograph of a nanopowder.

FIG. 3(b) show final mm-cm sized pellets. Regarding the production ofthe magnetic pellets, relatively inexpensive metallic salts may bedissolved in water and crystallized. The resulting magneticnanoparticles may be imaged by AFM microscopy and TEM microscopy (seeFIGS. 5-6).

FIG. 16 shows an image of an SrFe₁₂O₁₉ powder obtained with an opticalmicroscope. Nano powder of SrFe₁₂O₁₉ is characterised by having ared/brown luster—in contrast to larger sized particles and pressedpellets, which are black.

The following Examples document how to successfully align very smallnanoparticles of SrFe₁₂O₁₉, such as significantly smaller than 0.1 μm,by using the anisotropic shape of the nanoparticles. The nanopowdershave an aspect ratio of 1/10 between the dimensions along the a/b-axesand the c-axis. Applying an elevated pressure and temperature to thesenanoparticles causes alignment of the nanopowders without applyingmagnetic field or using organic solvents. The elevated temperaturefacilitates breaking the magnetic interaction and the uniaxial forcefield facilitates in aligning the platelet shaped nanoparticles. Themethod proves to produce bulk samples with high energy product, almost2.5 times larger than that of a conventional pressed powder, treated bythe same pressing method. By ‘conventional pressed powder’ is understoodconventional powder treated, such as pressed, by the same method, suchas conventional powder pressed according to Example II. The preparationof platelet-shaped nanoparticles and pressing them into bulk sampleswith high energy product using neither magnetic field nor organicsolvents presents a method for producing improved magnets. The processconsist of two steps, synthesis of nanoparticles from usingsupercritical synthesis and spark plasma sintering pressing of theobtained powder samples to produce dense pellets with high energyproduct.

Production of Nanoparticles and Compaction into Magnets

The atomic structure including size and shape of the nanoparticles maybe controlled in a single synthesis step. In one exemplary productionmethod, cheap metallic salts are dissolved in water and crystallized byfast heating, such as very fast heating, and subsequent cooling. By veryfast heating may be understood more than 20 K/second.

To obtain the right crystalline product a number of parameters may beaccurately controlled. For example, such parameters may include thereactor pressure, temperature, heating rates, and reaction time.Furthermore, other relevant parameters may be relevant, such as pH,concentration, stoichiometry, and nature of the ingredients of the metalion solution.

In an exemplary embodiment, the synthesis may be monitored continuouslyusing X-ray diffraction as a function of time, which gives insight intoat least some of the growth parameters.

Example I Supercritical Synthesis

Thin platelets of SrFe₁₂O₁₉ (size ˜30×30×3 nm³) were prepared throughsupercritical synthesis, where iron and strontium nitrates(Fe(NO₃)₃.9H₂O and Sr(NO₃)₂) are dissolved in deionized water to obtainprecursor solutions with a Sr:Fe ratio of 1:1. The concentration was0.05 M. NaOH was added in double concentration with respect to nitratesfrom both Fe(NO₃)₃.9H₂O and Sr(NO₃)₂. The alkaline solution was addeddrop wise under constant stirring and resulting in a dark redprecipitate. The precursor solution containing the precipitate was fedinto the supercritical reactor from a 200 mL injector and pumped intothe system at the flow rate of 5 mL/min. Deionized water was feed from asecond line at a flow rate of 15 mL/min. Precursor and supercriticalwater meet at the mixing point, see FIG. 4.

FIG. 4 shows a schematic diagram of the hydrothermal flow synthesissetup. The illustration is extracted from the reference“Glucose-assisted continuous flow synthesis of Bi ₂ Te ₃ nanoparticlesin supercritical/nearcritical water”, by Mi, J. L., et al., Journal ofSupercritical Fluids, 2012. 67: p. 84-88, which reference is herebyincorporated by reference in entirety.

At the mixing point the nucleation is initiated and crystallizationtakes place as the precipitates flow through the reactor. The product isquenched by a cold water jacket and collected at the outlet through apressure relief valve. The reaction temperature was 390° C. and thesystem was pressurized to 250 bars. The obtained product was washed,centrifuged and dried. The resulting nanoparticles are very small withan average size of ˜30×30×3 nm³ as extracted from powder X-raydiffraction, TEM and AFM pictures. The magnetic properties of the assynthesized compound were investigated using a vibration samplemagnetometer (VSM). FIG. 4 shows the supercritical synthesis apparatus.Images of TEM and AFM are shown in FIGS. 5-6. The images reveal thedimension of the samples along with a regular hexagonal shape of thenanoparticles, with relatively short c-axis compared to the a- andb-axes.

FIGS. 5-6 show nanoparticles of SrFe₁₂O₁₉ prepared by the supercriticalflow synthesis method. Both images show agglomerates SrFe₁₂O₁₉nanoparticles.

FIG. 5 shows a Transmission Electron Microscopy (TEM) image.

FIG. 6 shows an Atomic Force Microscopy (AFM) image.

Finally the powder diffraction pattern is shown in FIG. 7, Rietveldrefinements have been used to model the crystal structure along with thesize and shape of the nanoparticles.

FIG. 7 shows the powder diffraction pattern of the supercriticalsynthesized sample, the grey data points (716) represent the observeddata (Y_(OBS)), the black line (722) represent the Rietveld model(Y_(CALC)) and the lower grey line (726) represent the difference(Y_(OBS)-Y_(CALC)) between the observed data and the model. The verticallight grey lines (724) are signifying the Bragg positions.

FIG. 8 shows a hysteresis loop (816) of as prepared SrFe₁₂O₁₉. Themagnetic properties of the as prepared nanoparticles are shown in FIG.8. Before measuring the “as prepared” sample it was cold compacted andglued with cement to a VSM sample stick—the cement is also beneficial inorder to bind the nanoparticles and prevent reorientation of themagnetic grains. The “as prepared sample” has a saturation magnetisationof 30 emu/g, a remenance of 11 emu/g, and a coercivity of 1032 Oe.

Example IV 1^(st) Alternative Supercritical Synthesis

A study of the effect of temperature, pH and concentration demonstratesthat the correct phase (SrFe₁₂O₁₉) can be made at reaction temperatureof 350° C. The pH have a minor effect and the compound can be made withKOH (instead of NaOH). Other parameters are the same as in EXAMPLE I.The effect of concentration, when adding the base is remarkable and fromthe table below (TABLE I) it can be seen how the concentration ofFe(NO₃)₃.9H₂O, when adding the base determines the sizes of the preparedparticles, such as the prepared nanocrystallites.

TABLE I Fe(NO₃)₃•9H₂O (M)* ab (nm) c (nm) ab/c ratio 0.05 19.1 2.5 7.640.075 29.4 4.0 7.35 0.150 41.0 5.5 7.45 0.300 62.1 7.7 8.06 0.500 69.09.4 7.34 0.750 62.9 11.2 5.61 *Concentration of Fe(NO₃)₃•9H₂O in theprecursor solution at the moment of addition of the base. All solutionswere, after the addition of the base, taken to a final Fe(NO₃)₃•9H₂Oconcentration of 0.05M by adding water.

FIG. 17 shows a graph of particle size as a function of concentration ofFe(NO₃)₃.9H₂O and Sr(NO₃)₂ when addition of the base is taking place. Ascan be observed, with increasing concentration of Fe(NO₃)₃.9H₂O andSr(NO₃)₂ when addition of the base is taking place, larger particles areformed. The upper curve with square markers represents the size alongthe ab-axis. The lower curve with circle markers represents the sizealong the c-axis.

Example V 2^(nd) Alternative Supercritical Synthesis

The possibility of making BaFe₁₂O₁₉ nanoparticles using supercriticalflow has also been demonstrated, and here size can be adjusted byvarying the Ba to Fe ratio of the precursor solution. Other parametersare the same as in EXAMPLE I. The results show, as equivalently observedwith SrFe₁₂O₁₉, an increase of particle size with increasing amount ofBa with respect to Fe. TABLE II shows particle sizes for various sample(where each row corresponds to a sample), where the ratio in the firstcolumn corresponds to the ratio of Ba with respect to Fe.

TABLE II SAMPLE ab (nm) c (nm) ab/c 1-1 25.8 3.6 7.1 1-2 32.8 5.2 6.41-4 46.8 7.5 6.3 1-6 51.2 8.1 6.4 1-10 83.5 12.5 6.7

FIG. 18 shows a graph of particle size as a function of the ratio of Bawith respect to Fe. The upper curve with square markers represents thesize along the ab-axis. The lower curve with circle markers representsthe size along the c-axis.

In all cases within Example V, addition of NaOH 16 M was done on aprecursor solution with Fe(NO₃)₃.9H₂O concentration of 0.05 M. No morewater was added after the base, therefore being 0.05 M the finalconcentration of Fe(NO₃)₃.9H₂O in the precursor solution. Theconcentration of Ba(NO₃)₂ was changed accordingly to result in thepresented ratios.

Example II Spark Plasma Sintering

The prepared platelet nanoparticles are pressed into compact magnets bymeans of spark plasma sintering (SPS) press. The nanoparticles areprepared according to Example I. The nanopowder is loaded into agraphite pressing tool and uniaxial pressure is applied to the punches,meanwhile a pulsed DC current is directed through the graphite pressingtool and the sample.

FIG. 10 is a schematic illustration of the working principles of a sparkplasma sintering press.

FIG. 11 illustrates the proposed process, when the thin platelets areexposed to elevated pressure and temperature.

The SrFe₁₂O₁₉ nanopowder was loaded into the graphite pressing tool awith an inner diameter of 8 mm and 0.3 g were loaded into thecylindrical cavity and inserted into the SPS vacuum chamber. A minimalpressure was applied to hold the pressing tool in place. The chamber wassealed and the atmosphere was removed from the chamber. A pressure of 80MPa was applied to densify the sample. A direct pulsed DC current wasapplied and the sample was heated by an approximate rate of 100° C./minto a temperature of 950° C., the temperature was held for 2 minutesbefore cooling to room temperature. FIG. 11 shows an illustration ofwhat is expected to happen as high pressure and temperature is applied.

Powder diffraction pattern of the pellets reveals a significantalignment of the crystallites in the sample and growth of thenanoparticles.

FIG. 12 shows normalized powder diffraction pattern for as preparednanopowder (1216), SPS pressed commercial powder (1220) and SPS pressednanopowder (1218). Index is given for some of the main reflections. SPSpressed is understood to refer to pressing according to Example II. Forthe SPS nanopowder (1218), such as the SPS pressed nanopowder (1218), itis clear that the most pronounced peaks are associated with (00l) givendirect information on the preferred orientation of the crystal grainsinside the SPS pellet. The index on the powder diffraction pattern foras prepared nanopowder (1216) shows that the reflections originatingfrom a/b axes is much narrower than the other peaks point to the largerdimensions along the a/b axis. The powder diffraction pattern forcommercial sample (1220) does not show extraordinary alignment. FIG. 12is a comparison between the as prepared nano particles and the nanoparticles after SPS pressing. From the peak width the growth along thec-axis can be estimated to increase from about 3 nm to about 60 nm. Theparticle size in the a/b direction is currently being investigated. Themost pronounced feature for the SPS nanopowder is the high degree ofpreferred orientation. The peaks observed are all characterized byhaving an l-index, (e.g. 00l, where 00l=004, 006, 008 etc), thereforethe crystallite grains have preferential orientation with the c-axislattice planes parallel to the surface. In other words the platelets arepreferentially ordering with the c-axis planes perpendicular to thepressing direction.

The obtained pellet from SPS pressing was 8 mm diameter and 1 mm thick.A diamond blade saw was used to cut a piece with mass of 12.34 mg fromthe original disc. The sample was heated to 200° C. in vacuum for 1 hourto remove moisture before being weighted and mounted for VSMmeasurements.

FIG. 9 shows a comparison of different samples, in particular the asprepared (which in the figure is labelled ‘As synthesized’) samples(916), the as prepared sample after SPS Pressing (which in the figure islabelled ‘SPS nanopowder’) (918) and commercial sample (which in thefigure is labelled ‘SPS commercial’) (920) which is a commerciallyavailable SrFe₁₂O₁₉ powder pressed in the same way as the SPS pressed“as prepared” nanopowder, such as pressed according to Example II. Thehysteresis curve in FIG. 9 shows the intensity of magnetization asfunction of the applied magnetic field. The external field was scannedfrom −1.5 to 1.5 T. Based on the hysteresis curves the energy productBH_(max) can be calculated. The as prepared powder has a BH_(max) of 1kJ/m³, while the commercial sample (which commercial sample has beenobtained from TRIDELTA Hartferrite GmbH, type 14T) has a value of 11kJ/m³ and the aligned SPS pressed nanopowder reveals an energy productof 26 kJ/m³. The SPS pressed “as prepared” sample has a saturationmagnetisation of 69 emu/g, a remanence 57 emu/g, and a coercivity of3788 Oe. The SPS pressed commercial sample has a saturationmagnetisation of 62 emu/g, a remenance of 38 emu/g, and a coercivity of3784 Oe.

The final size of the particles within the permanent magnetic materialafter SPS pressing have been extracted from data measured at beamlineBL44XU, SPring8, Japan. The data extends to much higher scatteringvalues than our data obtained from a standard X-ray source, such as astandard Rigaku X-ray diffractometer. The size (determined from X-raydiffraction measurements conducted at SPring8) along the a and b-axes isapproximately 70 nm, and the size along the c-axis is approximately 60nm. From a Rigaku X-ray diffractometer we estimated the size along thec-axis size to be 58 nm, i.e., in very good agreement with thesynchrotron data from Spring8.

FIGS. 13-14 show the refinements of the full data (FIG. 13) and the datacorresponding approximately to the range covered with the Rigaku X-raydiffractometer (FIG. 14). It can be observed that the peaks aresignificantly sharper—than those shown for “as prepared sample” (seeref. sign 716 in FIGS. 7 and 1216 in FIG. 12) confirming that growth hastaken place. In an embodiment still larger particles may be obtained viaannealing, for e.g., 1 hour, such as 2 hours, such as 3 hours.

The relative orientation of particles within the permanent magneticmaterial may be determined by taking intensities (in a powder X-raydiffraction spectrum) for reflections along the 00l direction and divideby the total intensity of all reflections as described elsewhere in thisdocument, such as the relative orientation being given by the ratio:

ΣI(00l)/ΣI(hkl),

cf., the relative orientation quantification method described elsewherein this application. This equation gives a comparable number for therelative orientation in different samples, such as permanent magneticmaterials. For a random orientation sample probed at the Spring8, therelative orientation is given as ΣI(00l)/ΣI(hkl)=4%. For thecommercially available powder compacted with SPS the relativeorientation is 9%. The permanent magnetic material according to anembodiment of the present invention obtains 55%. Comparing these numbersit is clear that the inventive permanent magnetic material issignificantly more aligned than the normal commercial sample and thatthe commercial sample is more aligned than the completely random sample.

To sum up on examples I-II: The Examples demonstrate an exemplary methodfor producing high energy product (BH_(max)) SrFe₁₂O₁₉ pellets. The highenergy product comes about from the alignment of platelet shapedSrFe₁₂O₁₉ nanoparticles by applying a uniaxial pressure and elevatedtemperature. The applied pressure and temperature causes thenanoparticles to align and grow into more optimal sizes domains comparedto the as prepared nanoparticles. Our nano based magnets show an energyproduct almost 2.5 times larger than conventional prepared and pressedhexaferrite. Thus, it is shown how to prepare highly aligned bulkmagnets from nanoparticles with small grains without the use of magneticguide field or organic solvents, using a process of aligningnanoparticles using uniaxial pressure combined with heating for thepreparation of strong permanent magnetic materials based on hexaferritematerial.

Example VI Spark Plasma Sintering with Pre-Heating

In this example, pre-heating has been applied, i.e., the temperature ofthe particles has been raised before pressing has been initiated.

A permanent magnetic material has been prepared according to Example VIby successively:

-   -   heating (i.e., pre-heating) the as prepared nanopowder        (corresponding to the particles Example IV prepared with a        Fe(NO₃)₃.9H₂O concentration of 0.15 M) to 600° C., wherein said        heating (i.e., raising the temperature from room temperature to        600° C.) takes 2 minutes,    -   holding the said nanopowder for 1 minute at this temperature        (600° C.),    -   then applying a pressure of 100 MPa and subsequently,    -   heating to 1000° C. while still maintaining the pressure of 100        MPa, wherein said heating (i.e., raising the temperature from        600° C. to 1000° C.) takes 4 minutes,    -   holding the said nanopowder for 2 minute at this temperature        (1000° C.),        wherein the total pressing procedure takes 9 minutes, including        heating from 600° C. to 1000° C. which takes 4 minutes, and        holding the temperature at 1000° C. for 2 minutes.

This procedure results in excellent alignment of the nanoparticles.

Applying heating before applying pressure is an advantage in relation tothe alignment as the resulting relatively high temperature mayfacilitate reducing or eliminating magnetic interaction between theparticles and thus make the nanoparticles more susceptible to appliedpressure, which may in turn improve the alignment of the samples. It isunderstood when referring to ‘applying heating before applyingpressure’, that ‘before’ may be understood as ‘immediately before’and/or as ‘applying heating before applying pressure so that the samplehas an elevated temperature, such a temperature above room temperature,such as a temperate of at least the blocking temperature, when theapplication of pressure is initiated’.

The magnetic material prepared by SPS pressing at 100 MPa and 1000° C.of particles prepared (corresponding to the particles prepared accordingto Example IV) from of Fe(NO₃)₃.9H₂O concentration of 0.15 M and whereheating (to 600° C.) prior to applying pressure took place have asaturation magnetisation of 70 emu/g, a remanence magnetization of 61emu/g, and a coercivity of 2829 Oe. This gives rise to an energy productBH_(max) of 25 kJ/m³. This is comparable to the results obtainableaccording to particles obtained as in Example I and compacted accordingto Example II, such as with a precursor concentration of 0.05 MFe(NO₃)₃.9H₂O and pressing at 80 MPa and heating to 950° C.

To sum up on examples I-VI: It is demonstrated that alignment of largerparticles (this is with concentration 0.15 M for their startingprecursor) is possible. Furthermore, observations indicate that apermanent magnetic material according to Examples IV and IV may resultin a very well aligned alignment structure. Furthermore, the Examplesgoes to show that within some range of particle sizes and within somerange of pressing parameters, strong magnets can be produced.

Example III Effect of Annealing

A series of annealing tests on a sample that was SPS pressed (accordingto Example I and II) is carried out. This is done by cutting differentsamples from the same pellet—i.e. all conditions are kept the sameexcept the time in which the sample has been sitting in the furnace. TheFurnace was heated to 750° C. and samples was inserted and furtherheating to 850° C. this was done within 1 hour. The samples were thenheld at different times as shown in FIG. 15.

FIG. 15 shows the effect on the energy product of annealing at 850° C.at different time periods.

The annealing process allows the nanoparticles to grow. From FIG. 15 itcan be seen that 4 hours give an optimum with an energy product of about28.5 kJ/m³. Longer times cause the energy product to steadily decrease,such as longer times until 4 hours cause the energy product to steadilydecrease (after which it starts dropping).

Samples prepared with different annealing time gives approximate a/bdimensions of 105 nm and c-axis of 70 nm from data collected at Spring8.Data collected with a Rigaku Smartlab corresponds within a small marginwith this and gives a/b dimensions of 120 nm and c-axis of 60 nm.

Example VII Providing Pole Figures and Texture Index

Pole figures reveal very high alignment of the nanoparticles afterpressing according to Examples II and VI.

A method for estimating grain alignment is provided by measurement ofpole figures. From pole figures an alignment distribution can bedetermined. In other words, what fraction of a sample is aligned withthe c-axis parallel to the flat pellet surface.

FIG. 19 is a schematic of the pole figure measurement.

The pole figure is measured by varying the incident beam with respect tothe surface of the sample. Practically this is done by rotating thesample around the chi axis (chi is the axis defined as the line betweenthe source and the detector, chi axis is shown as a dashed line in FIG.19). The chi angle is step at 5° and at every chi—the sample is rotatedaround the phi axis (the phi axis corresponds to the vector normal tothe sample surface, the phi axis in FIG. 19 coincides with the (008)reflections). The pole figure information is collected at thereflections (110), (008), (107), (114), and (203).

FIG. 21 shows reduced pole figures resulting from obtaining pelletsaccording to Example I (FIGS. 21A-B) or Example IV prepared with aFe(NO₃)₃.9H₂O concentration of 0.15 M (FIGS. 21C-E) and 0.750 M (FIG. 21F) compacting according to various methods, more particularly (TI refersto ‘texture index’):

FIG. 21(A): cold pressing (resulting in TI=1.60),

FIG. 21(B): direct pressing according to Example II (resulting inTI=3.25),

FIG. 21(C): pressing according to Example VI (resulting in TI=11.7),

FIG. 21(D): pressing according to Example VI (resulting in TI=11.6),

FIG. 21(E): pressing according to Example VI (resulting in TI=8.65),

FIG. 21(F): pressing according to Example VI (resulting in TI=17.2).

FIG. 21 shows that the SPS compacted samples in FIG. 21B-F comprisesparticles, which are aligned to a larger degree compared with the coldcompacted sample in FIG. 21(A), which hardly reveals any preferredalignment. The maximum (“max”) values indicated in the subfiguresrelates to the texture, since is shows the height of the peak. Theminimum values (“min”) gives information about the randomly orientedsample or the background level.

By cold pressing is understood a sample obtained as a cold compactedsample, more particularly a sample (i.e., particles) which was (cold)compacted (into a permanent magnetic material) by pressing at roomtemperature.

The obtained data is feed into the MTEX software (version 3.4.1) and theorientation distribution function (ODF) is calculated, from the ODF itis possible to extract the texture index (TI) and the reduced polefigure, which takes all collected data into account. Examples of reducedpole figures are shown in FIG. 21—along with the texture index for thedifferent samples. The program MTEX is obtained from the web-address:https://code.google.com/p/mtex/. The following references describe theusage and the algorithms behind MTEX: A novel pole figure inversionmethod: specification of the MTEX algorithm, Hielscher, Schaeben: J. ofAppl. Cryst. (2008), 41(6) (which is hereby included by reference inentirety) and Orientation Distribution Within a Single Hematite Crystal,R. Hielscher, H. Schaeben, H. Siemes: Math. Geosci. (2010), 42, 395-375(which is hereby included by reference in entirety).

The MTEX software also allows extraction of the volume fraction of thesample with a specific alignment. This is shown in FIG. 20.

FIG. 20 shows the volume fraction of samples with a given alignment,more particularly the fraction of the sample found within a 5° angle.The cold pressed sample (corresponding to the sample in FIG. 21(A)) isrelatively close to having random orientation. This is also seen fromthe texture index of 1.60 being close to the value for a random powderof 1. The direct SPS pressed sample (corresponding to the sample in FIG.21(B)) shows somewhat better sample alignment, texture index 3.25,however with a broad maximum. The Preheated SPS samples (correspondingto the samples in FIGS. 21(C-F)) have a significantly narrow alignmentof the grains and the texture indexes in all cases are above 8. Thecurves representing samples manufactured using pre-heating are providedwith filled markers. The curve representing the direct SPS pressedsample is provided with a non-filled, square marker. The curverepresenting the cold pressed sample is provided with a non-filled,round marker.

Example VIII Sample Alignment from Magnetic Measurements

An estimate of the samples alignment can also be extracted from orientedmagnetization measurements collected in a vibrating sample magnetometer(VSM).

FIG. 22 shows the sample setup in the VSM. The sample is held betweentwo quartz rods placed inside a non-magnetic brass holder. The samplealignment with respect to the applied field (H) (i.e., the externalmagnetic field) gives the easy axis. For the SPS pressed samples theeasy axis is coinciding with the surface normal of the pressed pellets.In the left sub-figure (marked 0°) the easy axis (indicated with arrow2228) is aligned with respect to external magnetic field. In this casethe most square-shaped hysteresis curve is obtained. In the rightsub-figure (marked 90°) the sample has been rotated 90°, so that theeasy axis (indicated with arrow 2230) is aligned perpendicularly (withan angle of 90°) with respect to the applied field. This 90° rotationproduces the hard magnetic axis. If the sample is perfectly aligned, the0° measurement should produce a square hysteresis curve, while the 90°should produce a curve with polarization (J) as a function of magneticfield (H), where the curve goes through the origo (0, 0). By taking theratio of the remanence values J_(r)(0°)/J_(r)(90°) obtained for the twosituations, an estimate of the sample alignment can be extracted. For acompletely isotropic magnet J_(r)(0°)/J_(r)(90°)=1, while for an alignedanisotropic magnet J_(r)(0°)/J_(r)(90°)>1, and for a perfectly alignedanisotropic magnet)) J_(r)(0°)/J_(r)(90°) goes towards infinity.

FIG. 23 shows hysteresis curves for the sample labelled ‘SPS48’(preheated) (a sample where particles are prepared according to ExampleIV (with a Fe(NO₃)₃.9H₂O concentration of 0.75 M), and where pressing iscarried out according to Example VI) rotated in steps of 15 degrees from0° to 90°. In this case the ratio of J_(r)(0°)/J_(r)(90°)=4.35, and itis observed how the curvature of the hysteresis curve in the secondquadrant increases with increasing angle. For a sample with directpressing (where particles are obtained according to Example I andpressing is done according to Example II) the ratioJ_(r)(0°)/J_(r)(90°)=3.03. The aligned magnetization measurements allowan independent estimate of the sample alignment.

Example IX Magnetic Measurements with Differently Aligned Samples

FIG. 24 shows magnetization measurements of differently preparedSrFe₁₂O₁₉ nanoparticles with respect to sample preparation and pressingconditions, more particularly hysteresis curves for pellets pressedunder different conditions and with different alignment. The coldpressed sample, as well as samples labelled ‘SPS27’, ‘SPS45_2h’ and‘SPS47’ have been prepared from nanoparticles with sizes of about(a×b×c)=(30×30×3 nm³), while SPS48 has been prepared from largerparticles with sizes of (a×b×c)=(63×63×11 nm³). The particles for thecold pressed sample are prepared according to Example I. SPS27correspond to a sample prepared according to Examples I-II. Each ofSPS45_2h and SPS47 comprise particles prepared according to Example IV(with a Fe(NO₃)₃.9H₂O concentration of 0.15 M) where pressing is carriedout according to Example VI. SPS48 comprise particles prepared accordingto Example IV (with a Fe(NO₃)₃.9H₂O concentration of 0.75 M) wherepressing is carried out according to Example VI. The cold pressedsamples shows a very smooth reduction of J in the second quadrant of thecoordinate system point to an unaligned samples, where as all SPSsamples shows a relative abrupt change in J. The sample SPS47 has abetter alignment compared to SPS27 (the squareness of the curve),however the coercivity (H_(c)) is lower. SPS48 is showing the bestalignment of all the samples, but the coercivity is significantlyreduced, which may indicate that smaller particles are preferable in thefinal pellet. The figures derivable from FIG. 24 are inserted in TABLEIII

TABLE III Jr (T) Hc (T) BHmax (kJ/m3) Cold press 0.1463 0.0991 2.67 SPS27 0.379 0.379 25.592 SPS 45_2h 0.341 0.348 20.816 SP S 47 0.389 0.27925.114 SP S 48 0.393 0.182 10.508

To sum up, there is presented a method for providing a permanentmagnetic material comprising hexagonal ferrites, which method does notnecessitate neither large magnetic fields nor organic solvents. Theproduced permanent magnetic materials have excellent properties, inparticular in terms of energy product, such as in terms of energyproduct and density. In further aspects, the invention relates toparticles for providing the permanent magnetic material, and acorresponding method of manufacture. In particular embodiments of theinvention the hexagonal ferrite is given by CaFe₁₂O₁₉, SrFe₁₂O₁₉ orBaFe₁₂O₁₉, such as given by SrFe₁₂O₁₉ or BaFe₁₂O₁₉.

In embodiments E1-E42 of the invention, there is presented:

-   -   E1.A method for preparing particles comprising hexagonal        ferrite, for a magnetic material, the method comprising        -   Forming a precursor solution comprising elements of the            hexagonal ferrite,        -   Feeding the precursor solution, such as a precursor solution            containing a precipitate, into a supercritical reactor, so            as to carry out a supercritical synthesis of the particles,            wherein the particles have an anisotropic shape and wherein            the size of the particles are smaller than or equal to a            size enabling individual particles to become single domain            magnets.    -   E2.A method according to embodiment E1, wherein the hexagonal        ferrite comprises XFe₁₂O₁₉, where X is an element selected from        the group comprising Strontium (Sr) and Barium (Ba).    -   E3.A method according to embodiment E2, wherein a step of        forming the precursor solution comprises        -   dissolving            -   a compound comprising iron (Fe), such as a compound                selected from the group comprising                -   i. iron nitrate, such as (Fe(NO₃)₃.9H₂O,                -   ii. iron chloride, such as FeCl₃, and                -   iii. iron sulphate, Fe(SO₄)₃,        -   and/or dissolving            -   a compound comprising strontium (Sr), such as a compound                selected from the group comprising:                -   i. strontium nitrate, such as Sr(NO₃)₂,                -   ii. strontium hydroxide, such as Sr(OH)₂, and                -   iii. strontium chloride, such as SrCl₂.    -   E4.A method according to embodiment E2, wherein a step of        forming the precursor solution comprises        -   dissolving iron nitrate, such as (Fe(NO₃)₃.9H₂O, and            strontium nitrate, such as Sr(NO₃)₂.    -   E5.A method according to any one of embodiments E3 or E4,        wherein the precursor solution has a Sr: Fe ratio of 1:1.    -   E6.A method according to any one of embodiments E3 or E4,        wherein an alkaline solution is added in a concentration being        at least 1.25 times, such as at least 1.50 times, such as at        least 2 times, such as 2 times, the concentration of nitrates        from both the iron nitrate and the strontium nitrate.    -   E7.A method according to embodiment E6, wherein the alkaline        solution comprises a substance selected from the group        comprising: NaOH, KOH and LiOH.    -   E8.A method according to embodiment E6, wherein the alkaline        solution is added drop wise under constant stirring until a dark        red precipitate is formed.    -   E9.A method according to embodiment E1, comprising feeding the        precursor solution, such as the precursor solution containing        the dark red precipitate, into a supercritical reactor.    -   E10. A method according to embodiment E9, wherein the precursor        solution is fed into the supercritical reactor at the flow rate        of within 0.5-50 mL/min, such as within 1-10 mL/min, such as 5        mL/min.    -   E11. A method according to embodiment E1, comprising feeding        deionized water into the supercritical reactor at a flow rate of        within 0.15-150 mL/min, such as within 3-30 mL/min, such as 15        mL/min.    -   E12. A method according to embodiment E1, comprising        -   feeding the precursor solution, such as the precursor            solution containing the dark red precipitate, into the            supercritical reactor, at a first flow rate,        -   feeding deionized water into the supercritical reactor at a            second flow rate,        -   wherein the ratio of the first flow rate and the second flow            rate is between 1:0.3 and 1:30, such as between 1:1 and            1:10, such as 1:3.    -   E13. A method according to embodiment E1, wherein the precursor        solution and the deionized water meet at a mixing point.    -   E14. Particles comprising hexagonal ferrite for a magnetic        material, wherein the particles have an anisotropic shape and        wherein a size of the particles are smaller than or equal to a        size enabling individual particles to become single domain        magnets.    -   E15. Particles according to embodiment E14, wherein the        hexagonal ferrite comprises XFe₁₂O₁₉, where X is an element        selected from the group comprising Strontium (Sr) and Barium        (Ba).    -   E16. Particles according to embodiment E14, wherein dimensions        of the particles may be described by dimensions along a first        crystal axis (a-axis), a second crystal axis (b-axis) and a        third crystal axis (c-axis), and wherein dimensions of the        particles are substantially larger along the first crystal axis        (a-axis) and/or the second crystal axis (b-axis) relative to a        dimension along the third crystal axis (c-axis).    -   E17. Particles according to embodiment E14, wherein dimensions        of the particles are at least 2 times larger, such as at least 5        times larger, such as at least 10 times larger, such as        substantially 10 times larger, along a first crystal axis        (a-axis) and/or a second crystal axis (b-axis) relative to a        dimension along a third crystal axis (c-axis).    -   E18. Particles according to embodiment E14, wherein a dimension        of the particles along a first crystal axis (a-axis) is        substantially 30 nm and wherein a dimension along a second        crystal axis (b-axis) is substantially 30 nm and wherein a        dimension along a third crystal axis (c-axis) is substantially 3        nm.    -   E19. Particles according to embodiment E14, wherein an energy        product (BH_(max)) of the particles is at least 0.1 kJ/m³, such        as at least 1.0 kJ/m³, such as substantially 1 kJ/m³.    -   E20. A method for preparing a permanent magnetic material        comprising hexagonal ferrite, the method comprising:        -   obtaining particles comprising hexagonal ferrite, which            particles have an anisotropic shape,        -   compacting the particles into a permanent magnetic material,            wherein the step of compacting the particles comprises            applying a pressure above atmospheric pressure and a            temperature above room temperature, and wherein a size of            the particles after the step of compacting are smaller than            or equal to a size enabling individual particles to become            single domain magnets, such as the size enabling individual            particles to become single domain magnets.    -   E21. A method according to embodiment E20, wherein the hexagonal        ferrite comprises XFe₁₂O₁₉, where X is an element selected from        the group comprising Strontium (Sr) and Barium (Ba).    -   E22. A method according to embodiment E20, wherein the particles        are enlarged during the step of compacting the particles.    -   E23. A method for preparing a permanent magnetic material        according to embodiment E20, wherein the step of compacting the        particles comprises uniaxial hot pressing.    -   E24. A method for preparing a permanent magnetic material        according to embodiment E20, wherein the step of compacting the        particles comprises spark plasma sintering (SPS), such as the        method comprising loading the particles into a graphite pressing        tool wherein uniaxial pressure is applied to punches and a        pulsed DC current is directed through the graphite pressing tool        and the particles.    -   E25. A method for preparing a permanent magnetic material        according to embodiment E20, wherein the pressure is at least 20        MPa, such as least 40 MPa, such as least 60 MPa, such as least        80 MPa, such as 80 MPa, such a below 100 MPa, such as between 20        MPa and 100 MPa.    -   E26. A method for preparing a permanent magnetic material        according to embodiment E20, wherein a pulsed DC current is        applied so as to heat the particles at a rate of at least 10°        C./min, such as substantially 100° C./min.    -   E27. A method for preparing a permanent magnetic material        according to embodiment E20, wherein a pulsed DC current is        applied so as to heat the particles to a temperature of at least        800° C., such as at least 875° C., such as 950° C., such as        below 1450° C., such as between 800° C. and 1450° C.    -   E28. A method for preparing a permanent magnetic material        according to embodiment E20, wherein the temperature above room        temperature is held for at least 1 minute, such as at least 2        minutes, such as substantially 2 minutes, such as at least 5        minutes, such as at least 10 minutes, before cooling to room        temperature.    -   E29. A method for preparing a permanent magnetic material        according to embodiment E20, wherein the step of obtaining        particles, comprises        -   preparing particles according to the independent method of            embodiment E1, or        -   obtaining particles according to the independent product            embodiment E14.    -   E30. A method for preparing a permanent magnetic material        according to embodiment E20, wherein the method further        comprises annealing the permanent magnetic material, such as        annealing for at least 1 hour, such as at least 2 hours, such as        at least 4 hours, such as 4 hours, such as between 1-10 hours,        such as between 2-6 hours, such as between 2.5-5 hours, such as        4 hours, such as annealing at a temperature of between 800-1000°        C., such as at a temperature of 850° C., such as annealing for 4        hours at 850° C.    -   E31. A permanent magnetic materialic material comprising        particles comprising hexagonal ferrite, wherein a size of the        particles are smaller than or equal to a size enabling        individual particles to become single domain magnets, such as a        size enabling individual particles to become single domain        magnets.    -   E32. A permanent magnetic materialic material according to        embodiment E31, wherein the hexagonal ferrite comprises        XFe₁₂O₁₉, where X is an element selected from the group        comprising Strontium (Sr) and Barium (Ba).    -   E33. The permanent magnetic materialic material according to        embodiment E31, wherein crystallites in the permanent magnetic        material have a preferential orientation, such as the        crystallites in the permanent magnetic material being        substantially aligned.    -   E34. The permanent magnetic materialic material according to        embodiment E31, wherein a relative orientation of crystallites        within the permanent magnetic material is at least 10%, such as        at least 20%, such as at least 30%, such as at least 40%, such        as at least 45%, such as 55%.    -   E35. The permanent magnetic material according to embodiment        E33, wherein the preferential orientation is with c-axis lattice        planes parallel to each other, such as orthogonal to a pressing        direction employed during a preparation of the permanent magnet.    -   E36. The permanent magnetic material according to embodiment        E31, wherein crystallites in the permanent magnetic material        have a length along a third crystal axis (c-axis) of less than        250 nm, such as less than 200 nm, such as less than 150 nm, such        as less than 100 nm, such as less than 75 nm, such as less than        50 nm, such as within 3 nm and 150 nm, such as within 3 nm and        100 nm, such as within 3 nm and 75 nm, such as within 25 nm and        75 nm, such as within 50 nm and 70 nm, such as approximately 60        nm.    -   E37. The permanent magnetic material according to embodiment        E31, wherein crystallites in the permanent magnetic material        have a length along a first crystal axis (a-axis) and/or second        crystal axis (b-axis) of less than 250 nm, such as less than 200        nm, such as less than 175 nm, such as less than 150 nm, such as        less than 100 nm, such as less than 80 nm, such as less than 50        nm, such as within 30 nm and 175 nm, such as within 50 nm and 90        nm, such as within 60 nm and 80 nm, such as within 65 nm and 75        nm, such as approximately 70 nm.    -   E38. The permanent magnetic material according to embodiment        E31, wherein the permanent magnetic material has an energy        product (BH_(max)) of more than 11 kJ/m³, such as more than 15        kJ/m³, such as more than 20 kJ/m³, such as more than 25 kJ/m³,        such as at least 26 kJ/m³, such as 26 kJ/m³, such as at least        28.5 kJ/m³, such as 28.5 kJ/m³.    -   E39. The permanent magnetic material according to embodiment        E31, wherein the permanent magnetic material is shaped into a        permanent magnet which has a diameter of 8 mm.    -   E40. The permanent magnetic material according to embodiment        E31, wherein the permanent magnetic material is shaped into a        permanent magnet which has a thickness of 1 mm.    -   E41. The permanent magnetic material according to embodiment        E31, wherein the permanent magnetic material has a density of at        least 2.0 g/cm³, such as at least at least 3.0 g/cm³, such as at        least at least 4.0 g/cm³, such as at least at least 4.5 g/cm³,        such as 4.70 g/cm³.    -   E42. A device for inter-converting between electrical energy and        kinetic energy, wherein the device comprises the permanent        magnetic material according to embodiment E31.

Although the present invention has been described in connection with thespecified embodiments, it should not be construed as being in any waylimited to the presented examples. The scope of the present invention isset out by the accompanying claim set. In the context of the claims, theterms “comprising” or “comprises” do not exclude other possible elementsor steps. Also, the mentioning of references such as “a” or “an” etc.should not be construed as excluding a plurality. The use of referencesigns in the claims with respect to elements indicated in the figuresshall also not be construed as limiting the scope of the invention.Furthermore, individual features mentioned in different claims, maypossibly be advantageously combined, and the mentioning of thesefeatures in different claims does not exclude that a combination offeatures is not possible and advantageous.

1. A method for preparing a permanent magnetic material comprisinghexagonal ferrite, the method comprising: obtaining particles comprisinghexagonal ferrite, which particles have an anisotropic shape and;compacting the particles into a permanent magnetic material; wherein thecompacting of the particles comprises applying a pressure aboveatmospheric pressure and a temperature above room temperature, andwherein a size of the particles after compacting are smaller than orequal to a size enabling individual particles to become single domainmagnets. 2-53. (canceled)
 54. The method according to claim 1, whereinthe hexagonal ferrite comprises XFe₁₂O₁₉, where X is an element selectedfrom the group consisting of Calcium (Ca), Strontium (Sr) and Barium(Ba).
 55. The method according to claim 1, wherein obtaining particlescomprising hexagonal ferrite, which particles have an anisotropic shape,comprises obtaining particles comprising hexagonal ferrite, wherein theanisotropic shape is a plate like shape, and wherein the size of theparticles is at most 100 nm.
 56. The method according to claim 1,wherein the method comprises reducing or breaking a magnetic interactionbetween the particles when compacting the particles and/or duringcompacting the particles, so as to allow alignment of the particles whencompacting the particles and/or during compacting the particles.
 57. Themethod according to claim 1, wherein the method comprises: pre-heatingof the particles, wherein said pre-heating comprises applying apre-heating temperature above room temperature to said particles beforecompacting the particles, so that a temperature of said particles is thepre-heating temperature when the compacting is initiated.
 58. The methodaccording to claim 1, wherein the particles are enlarged during the stepof compacting the particles.
 59. The method according to claim 1,wherein the compacting of the particles comprises uniaxial hot pressing.60. The method according to claim 1, wherein the compacting of theparticles comprises spark plasma sintering (SPS).
 61. The methodaccording to claim 1, wherein the pressure is at least 20 MPa.
 62. Themethod according to claim 1, wherein a pulsed DC current is applied soas to heat the particles at a rate of at least 10° C./min.
 63. Themethod according to claim 1, wherein the particles are heated to atemperature of at least 800° C.
 64. The method according to claim 1,wherein at least partially during compacting the particles into apermanent magnetic material: the particles are heated to a temperatureof at least 800° C., and the pressure is at least 20 MPa.
 65. The methodaccording to claim 1, wherein a pulsed DC current is applied to heat theparticles to a temperature of at least 800° C.
 66. The method accordingto claim 1, wherein the temperature above room temperature is held forat least 1 minute, before cooling to room temperature.
 67. The methodaccording to claim 1, wherein the obtaining of particles, comprises:preparing particles by: forming a precursor solution comprising elementsof the hexagonal ferrite, and feeding the precursor solution, into asupercritical reactor, so as to carry out a supercritical synthesis ofthe particles, wherein the particles have an anisotropic shape andwherein the size of the particles are smaller than or equal to a sizeenabling individual particles to become single domain magnets, orobtaining particles comprising hexagonal ferrite for a magneticmaterial, wherein the particles have an anisotropic shape and wherein asize of the particles are smaller than or equal to a size enablingindividual particles to become single domain magnets.
 68. The methodaccording to claim 1, wherein the method further comprises annealing thepermanent magnetic material.
 69. A permanent magnetic materialcomprising particles comprising hexagonal ferrite, wherein a size of theparticles are smaller than or equal to a size enabling individualparticles to become single domain magnets, and wherein the particleshave an anisotropic shape, and wherein crystallites in the permanentmagnetic material have a preferential orientation.
 70. The permanentmagnetic material according to claim 69, wherein impurities, in thepermanent magnetic material contribute to less than 3 wt %, or less than1.5 wt %.
 71. The permanent magnetic material according to claim 69,wherein dimensions of the particles are at least 2 times larger, along afirst crystal axis (a-axis) and/or a second crystal axis (b-axis)relative to a dimension along a third crystal axis (c-axis).
 72. Thepermanent magnetic material according to claim 69, wherein the hexagonalferrite comprises XFe₁₂O₁₉, where X is an element selected from thegroup consisting of Calcium (Ca), Strontium (Sr) and Barium (Ba). 73.The permanent magnetic material according to claim 69, wherein a textureindex of the permanent magnetic material is at least
 2. 74. Thepermanent magnetic material according to claim 69, wherein a ratioJ_(r)(0°)/J_(r)(90°) is at least is at least 2 wherein said ratioJ_(r)(0°)/J_(r)(90°) is a ratio between a first remanence valueJ_(r)(0°) obtained at a first orientation of the permanent magneticmaterial with respect to an external magnetic applied field, and asecond remanence value J_(r)(90°) obtained at a second orientation ofthe permanent magnetic material with respect to the applied externalmagnetic field, wherein the second orientation is orthogonal to thefirst orientation.
 75. The permanent magnetic material according toclaim 69, wherein a relative orientation of crystallites within thepermanent magnetic material is at least 10%.
 76. The permanent magneticmaterial according to claim 69, wherein a preferential orientation iswith c-axis lattice planes parallel to each other.
 77. The permanentmagnetic material according to claim 69, wherein crystallites in thepermanent magnetic material have a length along a third crystal axis(c-axis) of less than 250 nm.
 78. The permanent magnetic materialaccording to claim 69, wherein crystallites in the permanent magneticmaterial have a length along a first crystal axis (a-axis) and/or secondcrystal axis (b-axis) of less than 250 nm.
 79. The permanent magneticmaterial according to claim 69, wherein the permanent magnetic materialhas an energy product (BH_(max)) of more than 11 kJ/m³.
 80. Thepermanent magnetic material according to claim 69, wherein the permanentmagnetic material has a density of at least 2.0 g/cm³.
 81. A device forinter-converting between electrical energy and kinetic energy, whereinthe device comprises: a permanent magnetic material prepared by:obtaining particles comprising hexagonal ferrite, which particles havean anisotropic shape; and compacting the particles into a permanentmagnetic material; wherein the step of compacting of the particlescomprises applying a pressure above atmospheric pressure and atemperature above room temperature, and wherein a size of the particlesafter the step of compacting are smaller than or equal to a sizeenabling individual particles to become single domain magnets; or apermanent magnetic material comprising particles comprising hexagonalferrite, wherein a size of the particles are smaller than or equal to asize enabling individual particles to become single domain magnets, andwherein the particles have an anisotropic shape, and whereincrystallites in the permanent magnetic material have a preferentialorientation.
 82. A method for preparing particles comprising hexagonalferrite, for a magnetic material, the method comprising: forming aprecursor solution comprising elements of the hexagonal ferrite, andfeeding the precursor solution, into a supercritical reactor, so as tocarry out a supercritical synthesis of the particles, wherein theparticles have an anisotropic shape and wherein the size of theparticles are smaller than or equal to a size enabling individualparticles to become single domain magnets.
 83. The method according toclaim 82, wherein the supercritical synthesis comprises heating of theprecursor solution, and wherein said heating is achieved by raising thetemperature at a rate of at least 10° C./second.
 84. The methodaccording to claim 82, wherein a reaction time period during thesupercritical synthesis is 10 minutes or less.
 85. The method accordingto claim 82, wherein the hexagonal ferrite comprises XFe₁₂O₁₉, where Xis an element selected from the group consisting of Calcium (Ca),Strontium (Sr) and Barium (Ba).
 86. The method according claim 82,wherein the forming of the precursor solution comprises dissolving acompound comprising iron (Fe), and/or dissolving a compound comprisingstrontium (Sr).
 87. The method according to claim 82, wherein theforming of the precursor solution comprises dissolving iron nitrate, andstrontium nitrate.
 88. The method according to claim 82, wherein theprecursor solution has a X:Fe ratio of 1:1 or R_(x):1, where R_(x) is anumber 0.1-2 and, wherein the precursor solution has a Sr:Fe ratio of1:1.
 89. The method according to claim 82, wherein the method furthercomprises adding a base to the precursor solution, wherein aconcentration of Fe³⁺ iron(III) within the precursor solution whenadding the base is within 0.05-0.750 M.
 90. The method according toclaim 82, wherein the method further comprises adding a base to theprecursor solution, wherein a concentration of Fe³⁺ iron(III) within theprecursor solution when adding the base is within 0.05-0.750 M, andwherein a final concentration of the precursor is 0.05-0.50 M and isachieved through dilution with base and/or water.
 91. The methodaccording to claim 82, wherein the forming of the precursor solutioncomprises: dissolving iron nitrate, and/or dissolving a nitrate selectedfrom the group consisting of strontium nitrate, barium nitrate, andcalcium nitrate, wherein an alkaline solution is added in aconcentration being at least 1.00 times, 1.50 times, or 2 times theconcentration of nitrates.
 92. The method according to claim 82, whereinan alkaline solution is added in a concentration being at least 1.25times, the concentration of nitrates from both the iron nitrate and thestrontium nitrate.
 93. The method according to claim 91, wherein thealkaline solution comprises a substance selected from the groupconsisting of NaOH, KOH and LiOH.
 94. The method according to claim 91,wherein the alkaline solution is added drop wise under constant stirringuntil a dark red precipitate is formed.
 95. The method according toclaim 82, comprising feeding the precursor solution, into asupercritical reactor.
 96. The method according to claim 82, comprisingfeeding the precursor solution into a supercritical reactor, wherein theprecursor solution is fed into the supercritical reactor at the flowrate of within 0.5-50 mL/min.
 97. The method according to claim 82,comprising feeding deionized water into the supercritical reactor at aflow rate of within 0.15-150 mL/min.
 98. The method according to claim82, comprising: feeding the precursor solution into the supercriticalreactor, at a first flow rate, and feeding deionized water into thesupercritical reactor at a second flow rate, wherein the ratio of thefirst flow rate and the second flow rate is between 1:0.3 and 1:30. 99.The method according to claim 97, wherein the precursor solution and thedeionized water meet at a mixing point.
 100. Particles comprisinghexagonal ferrite for a magnetic material, wherein the particles have ananisotropic shape and wherein a size of the particles are smaller thanor equal to a size enabling individual particles to become single domainmagnets.
 101. Particles according to claim 100, wherein the hexagonalferrite comprises XFe₁₂O₁₉, where X is an element selected from thegroup consisting of Calcium (Ca), Strontium (Sr) and Barium (Ba). 102.Particles according to claim 100, wherein dimensions of the particlesmay be described by dimensions along a first crystal axis (a-axis), asecond crystal axis (b-axis) and a third crystal axis (c-axis), andwherein dimensions of the particles are substantially larger along thefirst crystal axis (a-axis) and/or the second crystal axis (b-axis)relative to a dimension along the third crystal axis (c-axis). 103.Particles according to claim 100, wherein dimensions of the particlesare at least 2 times larger, along a first crystal axis (a-axis) and/ora second crystal axis (b-axis) relative to a dimension along a thirdcrystal axis (c-axis).
 104. Particles according to claim 100, wherein adimension of the particles along a first crystal axis (a-axis) is within20-40 nm, and wherein a dimension along a second crystal axis (b-axis)is within 20-40 nm, and wherein a dimension along a third crystal axis(c-axis) is within 2-4 nm.
 105. Particles according to claim 100,wherein an energy product (BH_(max)) of the particles is at least 0.1kJ/m³.